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V X 



" But with regard to the material world, we can at least go so far as 
this — we can perceive that events are brought about not by insulated 
interpositions of Divine power, exerted in each particular case, but bj" the 
establishment of general laws." Wf 

Whewell: Bridgewater Treatise. | 

" The only distinct meaning of the word 'natural* is stated, fixed, or \ 
settled ; since what is natural as much requires and presupposes an intel- 
ligent agent to render it so, i. <?., to effect it continually or at stated times, 
as what is supernatural or miraculous does to effect it for once." 

BuTLEH : Analogy of Eeveakd Beltgion. 

" To conclude, therefore, let no man out of a weak conceit of sobriety, j 
or an ill-applied moderation, think or maintain, that a man can search too \ 
far or be too well studied in the book of God's word, or in the book of ( 
God's works ; divinity or philosophy ; but rather let men endeavour ns ' 
endless progress or proficience in both." ) 

Bacon : Advancement of Learning. ^ 

Dcwn, Beckenham, Kent, 

First Edition, November 26ih, I860, 
Sixtit JEdition, Jan, 1872. 


Additions and Corrections, to the Sixin EDiTioy .. Page xi-xii 

HisTOKiCAL Sketch xiii-xxi 

Introduction.. 1-4 


Variation under Domestication. 

Causes of Variability — Effects of Habit and the use or disute of Parts — 
' Correlated Variation — Inheritance — Character of Domestic Varieties 
— Difficulty of distinguishing between Varieties and Species — Origin 
of Domestic Varieties from one or more Species — Domestic Pigeons, 
their Differences and Origin — Principles of Selection, anciently fol- 
lowed, their Effects — Methodical and Unconscious Selection — Un- 
knov/n Origin of our Domestic Productions — Circumstances favour- 
able to Man's power of Selection 5-32 


Variation under Nature. 

Variability —- Individual differences — Doubtful species — Wide ranging, 
much diffused, and common species, vary most — Species of the large: 
genera in each country vary more fiequently than the species of the 
smaller genera — Many of the species of the larger genera resembk 
varieties in being very closely, but unequally, related to each other, 
and in having restricted ranges 33-47 


Struggle for Existence. 

Its bearing on natural selection — The term used in a wide sense — Geome- 
trical ratio of increase — Rapid increase of naturalised animals and 
plants — Nature of the checks to increase — Competition universal — 
Effects of climate — Protection from the number of individuals — 
Complex relations of all animals and plants throughout nature — 
Struggle for life most severe between individuals and varieties of th< 
same species : often severe between species of the same genus — The 
relation of organism to ovganisTi the most important of all rela- 
tions 48-61 




Natural Selection; or the Survival of the Ffi'test. 

Natural Selection — its power compared with man's selection — its power 
on characters of trifling importance — its power at all ages and on both 
sexes — Sexual Selection — On the generality of intercrosses between 
individuals of the same species — Circumstances favourable and unfa- 
vourable to the results of Natural Selection, namely, intercrossing, 
isolation, number of individuals — Slow action — Extinction caused by 
Natural Selection — Divergence of Character, related to the diversity of 
inhabitants of any small area, and to naturalisation — Action of Natural 
Selection, through Divergence of Character and Extinction, on the 
descendants from a common parent — Explains the grouping of all 
organic beings — Advance in organisation — Low forms preserved 
— Convergence of character — Indefinite multiplication of species — 
Summary P'^g<^ 62-105 


Laws of Variation. 

Effects of changed conditions — Use and disuse, combined with natTiral 
selection ; organs of flight and of vision — Acclimatisation — Correlated 
variation — Compensation and economy of growth — False correlations i 
— Multiple, rudimentary, and lowly organised structures variable — ■ J 
Parts developed in an unusual manner are highly variable : specific 
characters more variable than generic : secondary sexual characters 
variable — Species of the same genus vary in an analogous manner — 
Rerersions to long-lost characters — Summary 106-132 


Difficulties of the Theory. 

Difliculties of the theory of descent with modification — Absence or rarity 
of transitional varieties — Transitions in habits of life — Diversified 
habits in the same species — Species with habits widely difierent 
from those of their allies — Organs of extreme perfection — Modes of 
transition — Cases of difficulty — Natura non facit saltum — Organs ■ 
of small importance — Organs not in all cases absolutely perfect — I 
The law of Unity of Type and of the Conditions of Existence embraced 
by the tlicory of Natural Se'-ection .. .. c. .. .. locJ-167 

CONTENTS. ''"11 


Miscellaneous Objections to the Theory of Natural 


Longevity — Modifications not necessarily simultaneous — Modification* 
apparently of no direct service — Progressive development — Characters 
of small functional importance, the most constant — Supposed incom- 
petence of natural selection to account for the incipient stages ot 
useful structures — Causes which interfere with the acquisition through 
natural selection of useful structures — Gradations of structure with 
changed functions — Widely different organs in members of the same 
class, developed from one and the same source — Reasons for disbeliev- 
ing in great and abrupt modi£cations Page 168-204 



Instincts comparable with habits, but different in their origin — Instincts 
graduated — Aphides and ants — Instincts variable — Domestic in- 
stincts, their origin — Natural instincts of the cuckoo, molothrus, 
ostrich, and parasitic bees — Slave-making ants — Hive-bee, its cell- 
making instinct — Changes of instinct and structure not necessarily 
simultaneous — Difficulties of the theory of the Natural Selection of 
instincts — Neuter or sterile insects — Summary .. .. 205-234 



Distinction betweon the sterility of first crosses and of hybrids — Sterility 
various in degree, not universal, affected by close interbreeding, re- 
moved by domestication — Laws governing the sterility of hybrids — ■ 
Sterility not a special endowment, but incidental on other differences, 
not accumulated by natural selection — Causes of the sterility of first 
crosses and of hybrids — Parallelism between the effects of changed 
conditions of life and of crossing — Dimorphism and Trimorphism — 
Fertility of va-rieties when crossed and of their mongrel offspring not 
Tiniversal — Hybrids and mongrels compared independently of their 
fertility — Summary 234-263 



On the Imperfectiox of the Geological Recoud. 

Ou the absence of intermedir-te varieties at the present day — On the 
nature of extinct intermediate varieties ; on their number — On 
the lapse of time, as inferred from the rate of denudation and ot 
deposition — On the lapse ci time as estimated by years — On the 
poorness of our palaeontological collections — On the intermittence of 
geological formations — On the denudation of granitic areas — On the 
absence of intermediate varieties in any one formation — On the sudden 
appearance of groups of species — On their sudden appearance in 
the lowest known fossiliferous strata — Antiquity of the habitable 
earth Page 264-28S 

-^ ^ , CHAPTER XL 

On the Geological Succession of Organic Beings. 

On the slow and successire appearance of new species — On their different 
rates of change — Speciesonce lost do-tiot-'reflppear — Groups of species 
follow the same general rules in their appearance and disappearance 
as do single species — On Extinction — On simultaneous changes in 
the forms of life throughout the world — On the affinities of extinct 
species to each other and to living species — On the state of develop- 
ment of ancient forms — On the succession of the same types within 
the same areas — Summary of preceding and present chapter 290-315 


Geographical Distribution. 

Ptvsent distribution cannot be accounted for by differences in physical 
conditions — Importance of barriers — AfTmity of the productions of 
the same continent — Centres of creation — Means of dispersal, by 
changes of climate and of the level of the laud, and by occasional means 
— Dispersal during the Glacial period — Alternate Glacial periods in 
the north and south 316-342 


Geographical Distribution — continued. 

Distribution of fresh-water productions — On the inhabitants of oceanic 
islands — Absence of Batrachians and of terrestrial Mammals — On 
the relation of the inhabitants of islands to those of the nearest main- 
land — On colonization from the nearest source with subsequent modi- 
fication — Summary of the last and present chapter .. .. 343-802 



Mutual Affinities of Organic Beings: Morphology: 
Embryology : Kudimentary Organs. 

Classification, groups subordinate to groups — JJftUuaX system — Rules 
and difficulties in classification, explained on the theory of descent 
with modification — Classification of varieties — Descent always used 
in classification — Analogical or adaptive characters — Affinities, 
general, complex, and radiating — Extinction separates and defines 
groups — MctRPHOLOGY, between members of the same class, between 
parts of the same individual — Embryology, laws of, explained by 
T'ariations not supervening at an early age, and being inherited at a 
corresponding age — Rudimentary organs ; their origin explained — 
Summary Page 363-403 



Recapitulation of the objections to the theary of Natural Selection — 
Recapitulation of the general and special circumstances in its favour 
. — Causes of the general belief in the immutability of species — 
How far the theory of Natural Selection may be extended — 
Effects of its adoption on the study of Natural History — Con- 
cluding remarks 404—429 

Glossary of Scientific TEiora 430 

Injsx ,, 44a 


The Diagram to front page 90, and to face the latter part of the Volume, 




Numerous small corrections have been made in tlie last 
and present editions on various subjects, according as the 
evidence has become somewhat stronger or weaker. The 
more important corrections and some additions in the pre- 
sent volume are tabulated on the following page, for the 
convenience of those interested in the subject, and who 
possess the fifth edition. JThe second edition was little 
more than a reprint of the first. The third edition was 
largely corrected and added to, and the fourth and fifth 
still more largely. As copies of the present work will 
be sent abroad, it may be of use if I specify the state of 
the foreign editions. The third French and second Ger- 
man editions were from the third English, with some few 
of the additions given in the fourth edition. x\ new fourth 
French edition has been translated by Colonel Moulinie ; 
of which the first half is from the fifth English, and the 
latter half from the present edition. A third German 
edition, under the superintendence of Professor Victor 
Cams, was from the fourth English edition ; a fifth is now 
preparing by the same author from the present volume. 
The second American edition was from the English second, 
with a few of the additions given in the third; and a 
third American edition has been printed from the fifth 
English edition. The Italian is from the third, the Dutch 
and three Eussian editions from the second English edition, 
and the Swedish from the fifth English edition. 

















































Chief Additions and Corrections. 

Influence of fortuitous destruction on natural selection 

On the convergence of specific forms. 

Account of the Ground-Woodpecker of La Plata modified. 

On tlie modification of the eye. 

Transitions through the acceleration or retardation of the 
period of reproduction. 

The account of the electric organ of fishes added to. 

Analogical resemblance between the eyes of Cephalopoda 
and Vertebi'ates. 

Chaparede on the analogical resemblance of the hair-clasp«rs 
of the Acaridff".. 

The probable use of the rattle to the Rattle-snake. 

Helmholtz on the imperfection of the human eye. 

The first part of this new chapter consists of portions, in a 
much modified state, taken from chap. iv. of the former 
editions. The latter and larger part is new, and relates 
chiefly to the supposed incompetency of natural selection 
to account for the incipient stages of useful structures. 
There is also a discussion on the causes which prevent 
in many cases the acquisition through natural selection 
of useful structures. Lastly, reasons are given for dis- 
believing in great and sudden modifications. Gradations 
of character, often accompanied by changes of function, 
are likewise here incidentally considei^. 

The statement with respect to young cuckoos ejecting their 
foster-brothers confirmed. 

On the cuckoo-like habits of the Molothrus. 

On fertile hybrid moths. 

The discussion on the fertility of hybrids not having been ac- 
quired through natural selection condensed and modified. 

On the causes of sterility of hybrids, added to and correctetL 

Pyrgoma found in the chalk. 

Extinct forms serving to connect existing groups. 

On earth adhering to the feet of migratory birds. 

On the wide geographical range of a species of Galaxias, 
a fresh-water fish. 

Discussion on analogical resemblances, enlarged and modified 

Homological structure of the feet of certain marsupial 

On serial homologies, corrected. 

Mr. E. Ray Lankester on morphology. 

On the asexual reproduction of Chironomus. 

On the origin of rudimentary parts, corrected. 

Recapitulation on the sterility of hybrids, corrected. 

Recapitulation on the absence of fossils beneath the Cani- 
brian system, corrected. 

Natural selection not the exclusive agency in the modi- 
fication of species, as always maintained in this work 

The belief in the separate creation of species ^^enei'ally held 
by naturalists, until a recent period. 




I WILL here a give a brief sketch of the progress of opinion on tne 
Origin of Species. Until recently the great majority of naturalists 
believed that species were immutable i^rouuctions, and had been 
separately created. This view has been ably maintained by many 
authors. Some few naturalists, on the other hand, have believed 
that species undergo modification, and that the existing forms of 
life are the descendants by true generation of pre-existing forms. 
Passing over allusions to the subject in the classical writers,* the first 
author who in modern times has treated it in a scientific spirit was 
Buffon. But as his opinions fluctuated greatly at different periods, 
and as he does not enter on the causes or means of the transforma- 
tion of species, I need not here enter on details. 

Lamarck was the first man whose conclusions on the subject 
excited much attention. This justly-celebrated naturalist first pub- 
lished his views in 1801 ; he much enlarged them in 1809 in his 
* Philosophic Zoologique,' and subsequently, in 1815, in the Intro- 
duction to his * Hist. Nat. des Animaux sans Vertebres.' In these 

♦ Aristotle, in his ' Physicce Auscultationes * (lib. 2, cap. 8, s, 2), after 
remarking that rain does not fall in order to make the corn grow, any 
more than it falls to spoil the farmer's corn when threshed out of doors, 
applies the same argument to organisation; and adds (as tianslated by 
Mr. Clair Grece, who first pointed out the passage to me), " So what hinders 
; the different parts [of the body] from having this merely accidental relation 
/ in nature ? as the teeth, for example, grow by necessity, the front ones 
/ sharp, adapted for dividing, and the grinders flat, and serviceable for mas- 
ticating the food ; since they were not made for the sake of this, but it 
n'as the result of accident. And in like manner as to the other parts in 
which there appears to exist an adaptation to an end. Whei'esoever, there- 
fore, all things together (that is all the parts of one whole) happened like 
as if they were made for the sake of something, these were preserved, 
having been appropriately constituted by an internal spontaneity ; and 
i whatsoever things wore not thus constituted, perished, and still perish." 
We here see the principle of natural selection shadowed forth, but how 
little Aristotle fully comprehended the principle, is shown by his remarks 
on the formation of the teeth. 


works he upholds the doctrine that all species, including man, are 
descended from other species. He first did the eminent service of 
arousing attention to the probability of all change in the organic, as 
well as in the inorganic world, being the result of law, and not of 
j_miraculous interposition. Lamarck seems to have been chiefly led 
to his conclusion on the gradual change of species, by the difficulty 
of distinguishing species and varieties, by the almost perfect gradation 
of forms in certain groups, and by the analogy of domestic produc- 
tions. With resp,ect to the means of modification, he attributed 
something to the direct action of the physical conditions of life, 
something to the crossing of already existing forms, and much to use 
and disuse, that is, to the effects of habit. To this latter agency he 
seems to attribute all the beautiful adaptations in nature ; — such as 
the long neck of the girafi'e for browsing on the branches of trees. 
But he likewise believed in a law of progressive development ; and 
as all the forms of life thus tend to progress, in order to account for 
the existence at the present day of simple productions, he maintains 
that such forms are now spontaneously generated.* 

Geoffrey Saint Hilaire, as is stated in his ' Life,' written by his 
son, suspected, as early as 1795, that what we call species are 
various degenerations of the same type. It was not until 1828 
that he published his conviction that the same forms have not been 
perpetuated since the origin of all things. Geoffrey seems to have 
relied chiefly on the conditions of life, or the " monde ambiant " as 
the cause of change. He was cautious in drawing conclusions, and 
did not believe that existing species are now undergoing modifica- 
tion ; and, as his son adds, " C'est done un problem e k reserver 
entierement h I'avenir, suppose meme que I'avenir doive avoir prise 
sur lui." 

* I have taken the date of the first publication of Lamarck from Isid. 
(jteoffroy Saint Hilaire's (' Hist. Nat. Ge'nerale,' torn, ii. p. 405, 1859) excel- 
lent history of opinion on this subject. In this work a full account is given 
of Buffon's conclusions on the same subject. It is curious how largely my 
grandfather, Dr. Erasmus Darwin, anticipated the views and erroneous 
grounds of opinion of Lamarck in his ' Zoonomia ' (vol. i. pp. 500-510), 
published in 1794. According to Isid. Geoffroy there is no doubt that 
Goethe was an extreme partisan of similar views, as shown in the Intro- 
duction to a work written m 1794 and 1795, but not published till long 
afterAvards : he has pointedly remarked (' Goethe als Naturforscher,' von 
Dr. Karl Meding, s. 34) that the future question for naturalists will be how, 
for instance, cattle got their horns, and not for what they are used. It is 
rather a singular instance of the manner in which similar views arise at 
about the same time, that Goethe in Germany, Dr. D -win in England, 
and Geoffroy Saint Hilaire (as we shaU immediately se^^ in France, came 
to the same conclusion on the origin of species, in the years 1794-5. 


In 1813, Dr. W. C. Wells read before the L'oyal Society ' An 
Account of a White Female, part of whose skin resembles that of 
a Negro ' ; but his paper was not published until his famous * Two 
Essays upon Dew and Single Vision ' appeared in 1818. In this 
paper he distinctly recognises the principle of natural selection, and 
this is the first recognition which has been indicated ; but he applies 
it only to the races of man, and to certain characters alone. After 
remarking that negroes and mulattoes enjoy an immunity from 
certain tropical diseases, he observes, firstly, that all animals tend to 
vary in some degree, and, secondly, that agriculturists improve their 
domesticated animals by selection ; and then, he adds, but what is 
done in this latter case " by art, seems to be done with equal efiicacy, 
though more slowl}'', by nature, in the formation of varieties of 
mankind, fitted for the country which they inhabit Of the acci- 
dental varieties of man, which would occur among the first few and 
scattered inhabitants of the middle regions of Africa, some one 
would be better fitted than the others to bear the diseases of the 
country. This race would consequently multiply, while the others 
would decrease ; not only from their inability to sustain the attacks 
of disease, but from their incapacity of contending with their more 
vigorous neighbours. The colour of this vigorous race I talse for 
granted, from what has been already said, would be dark. But the 
same disposition to form varieties still existing, a darker and a 
darker race would in the course of time occur : and as the darkest 
would be the best fitted for the climate, this would at length 
become the most prevalent, if not the only race, in the particular 
country in which it had originated." He then extends these same 
views to the white inhabitants of colder climates. I am indebted 
to Mr. Rowley, of the United States, for having called my atten- 
tion, through Mr. Brace, to the above passage in Dr. Well's A\-ork. 

The Hon. and Eev. W. Herbert, afterwards Dean of Manchester, 
in the fourth volume of the * Horticultural Transactions,' 1822, and 
in his work on the ' Amaryllidaceas ' (1837, p. 19, 339), declares that 
" horticultural experiments have established, beyond the possibility 
of refutation, that botanical species are only a higher and more per- 
manent class of varieties." He extends the same view to animals. 
The Dean believes that single species of each genus were created 
in an originally highly plastic condition, and that these have 
produced, chiefly by intercrossing, but likewise by variation, all 
our existing species. 

In 1826 Professor Grant, in the concluding paragraph in his 
u-ell known ^'iper (' Edinburgh Philosophical Journal,' vol. xiv. 
p. 283) on the Spongilla, clearly declares his belief that, species are 


descended from other species, and that the} become improred In 
the course of modification. This same view A^as given in his 55 lb 
Lecture, published in the ' Lancet ' in 1834. 

In 1831 Mr. Patricia Matthew published his work on * Naval 
Timber and Arboriculture,' in which he gives precisely the same 
view on the origin of species as that (presently to be alluded to) 
propounded by Mr. Wallace and myself in the ' Linnean Journal, 
and as that enlarged in the present volume. Unfortunately the 
view was given by Mr. Matthew very briefly in scattered passages 
in an Appendix to a work on a different subject, so that it remained 
unnoticed until Mr. Matthew himself drew attention to it in the 
* Gardeners' Chronicle,' on April 7th, 1860. The differences of Mr. 
Matthew's view from mine are not of much importance : he seems 
to consider that the world was nearly depopulated at successive 
periods, and then re-stocked ; and he gives as an alternative, that 
new forms may be generated " without the presence of any mould 
or germ of former aggregates." I am not sure that I understand 
some passages ; but it seems that he attributes much influence to 
the direct action of the conditions of life. He clearly saw, how- 
ever, the full force of the principle of natural selection. 

The celebrated geologist and naturalist. Von Buch, in his ex- 
cellent * Description Physique des Isles Canaries ' (1836, p. 147), 
clearly expresses his belief that varieties slowly become changed 
into permanent species, which are no longer capable of inter- 

Eafinesque, in his ' New Flora of North America,' published in 
1836, wrote (p. 6) as follows: — "All species might have been 
varieties once, and many varieties are gradually becoming species 
by assuming constant and peculiar characters ; " but farther on 
(p. 18) he adds, "except the original types or ancestors of the 

In 1843-44 Professor Haldeman (' Boston Journal of Nat. Hist. 
U. States,' vol. iv. p. 468) has ably given the arguments for and 
against the hypothesis of the development and modification of 
species : he seems to lean towards the side of change. 

The ' Vestiges of Creation ' appeared in 1844. In the tenth 
and much improved edition (1853) the anonymous author says 
(p. 155): — The proposition determined on after much considera- 
tion is, that the several series of animated beings, from the simplest 
and oldest up t:. the highest and most recent, are, under the provi- 
dence of God, the results, Jirstf of an impulse which has been 
imparted to the forms of life, advancing them, in definite times, by 
generation, through grades of oi'ganisation terminating in the 


highest dicotyledons and vertcbrata, these grades being few in 
DU"">ber, and generally marked by intervals of organic character, 
which we find to be a practical difficulty in ascertaining affinities ; 
second, of another impulse connected with the vital forces, tending, 
in the course of generations, to modify organic structures in accor- 
dance with external circumstances, as food, the nature of the 
habitat, and the meteoric agencies, these being the ' adaptations ' 
of the natural theologian." The author apparently believes that 
organisation progresses by sudden leaps, but that the effects 
produced by the conditions of life are gradual. He argues with 
much force on general grounds that species are not immutable 
productions. But I cannot see how the two supposed " impulses " 
account in a scientific sense for the numerous and beautiful co- 
adaptations which we see throughout nature ; I cannot see that we 
thus gain any insight how, for instance, a woodpecker has become 
adapted to its peculiar habits of life. The work, from its powerful 
and brilliant style, though displaying in the earlier editions little 
accurate knowledge and a great want of scientific caution, imme- 
diately had a very wide circulation. In my opinion it has done 
excellent service in this country in calling attention to the sub- 
ject, in removing prejudice, and in thus preparing the ground 
for the reception of analogous views. 

In 1846 the veteran geologist M. J. d'Omalius d'Halloy pub- 
lished in an excellent though short paper (* Bulletins de I'Acnd. Koy. 
Bruxelles,' tom. xiii. p. 581), his opinion that it is more probable 
that new species have been produced by descent with modification 
than that they have been separately created : the author first 
promulgated this opinion in 1831. 

Professor Owen, in 1849 (' Nature of Limbs,' p. 86), wrote as 
follows : — " The archetypal idea was manifested in the flesh under 
diverse such modifications, upon this planet, long prior to the 
existence of those animal species that actually exemplify it. To 
what natural laws or secondary causes the orderly succession and 
progression of such organic phenomena may have been committed^ 
we, as yet, are ignorant." In his Address to the British Association, 
in 1858, he speaks (p. li.) of "the axiom of the continuous 
operation of creative power, or of the ordained becoming of living 
things." Farther on (p. xc), after referring to geograpliical distri- 
bution, he adds, " These phenomena shake our confidence in the 
conclusion that the Apteryx of New Zealand and the Red Grouse 
of England were distinct creations in and for those islands resj^oc- 
tively. Always, also, it may be well to bear in mind that by the 
word 'creation' the zoologist means *a process he knows net 


what.' " He amplifies tliis idea by adding, that when snch cases 
as that of the Red Grouse are " enumerated "by the zoologist as 
evidence of distinct creation of the bird in and for such islands, he 
chiefly expresses that he knows not how the Eed Grouse came to 
be there, and there exclusively ; signifying also, by this mode of 
expressing such ignorance, his belief that both the bird and the 
islands owed their origin to a great first Creative Cause." If we 
interpret these sentences given in the same Address, one by the 
other, it appears that this eminent philosopher felt in 1858 his con- 
fidence shaken that the Apteryx and the Red Grouse first appeared 
in their respective homes, " he knew not how," or by some process 
"he knew not what." 

This Address was delivered after the papers, by Mr. Wallace and 
myself on the Origin of Species, presently to be referred to, had been 
read before the Linnean Society. When the first edition of this 
work was published, I was so completely deceived, as were many 
others, by such expressions as ** the continuous operation of creative 
j30wer," that I included Professor Owen with other palcBontologists 
as being firmly convinced of the immutability of species ; but it 
appears ('Anat. of Vertebrates,' vol. iii. p. 79G) that this was on 
my part a preposterous error. In the last edition of this work I 
inferred, and the inference still seems to me perfectly just, from a 
passage beginning with the words " no doubt the type-form," &c. 
(Ibid. vol. i. p. XXXV.), that Professor Owen admitted that natural 
selection may have done something in the formation of new 
species ; but this it appears (Ibid. vol. iii. p. 798) is inaccurate and 
without evidence. I also gave some extracts from a correspondenco 
between Professor Owen and the Editor of the * London Review,' 
from which it appeared manifest to the Editor as well as to myself, 
that Professor Owen claimed to have promulgated the theory of 
natural selection before I had done so ; and I expressed my surprise J 
and satisfaction at this announcement ; but as far as it is possible * 
to understand certain recently published passages (Ibid. vol. iii. 
p. 798), I have either partially or wholly again fallen into error. hI 

It is consolatory to me that others find Professor Owen's controver- 
sial writings as difficult to understand and to reconcile with each 
other, as I do. As far as the mere enunciation of the principle of 
natural selection is concerned, it is quite immaterial whether or 
not Professor Owen preceded me, for both of us, as shown in this 
historical sketch, were long ago preceded by Dr. Wells and Mr. 

M. Isidore Geoffrey Saint Hilaire, in his Lectures delivered in 
LSoO (of which a Resume appeared in the ' Revue et Mag. be ' 



Zoolog.,' Jan. 1851), briefly gives his reason for believing that 
specific characters " sont fixes, pour chaque espece, tant qu'elle se 
perpetue au milieu des memes circonstances : ils se modifient, si 
les circonstances ambiantes viennent k changer." " En resume, 
Vobservation des animaux sauvages ddmontre dejk la variabilite 
limitee des espfeces. Les experiences sur les animaux sauvages 
devenus domestiques, et sur les animaux domestiques redevenus 
sauvages, la de'montrent plus clairement encore. Ces mSmes exp(?- 
riences prouvent, de plus, que les difi'^rences produites peuvent etro 
de valeur genert'que''' In his * Hist. Nat. Generale ' (torn. ii. p. 
430, 1859) he amplifies analogous conclusions. 

From a circular lately issued it appears that Dr. Freke, in 1851 
(* Dublin Medical Press,' p. 322), propounded the doctrine that all 
organic beings have descended from one primordial form. His 
grounds of belief and treatment of the subject are wholly different 
from mine ; but as Dr. Freke has now (1861) published his Essay 
on * the Origin of Species by means of Organic Affinity,' the diffi- 
cult attempt to give any idea of his views would be superfluous 
on my part. 

Mr. Herbert Spencer, in an Essay (originally published in the 
* Leader,' March 1852, and republished in his * Essays ' in 1858), 
has contrasted the theories of the Creation and the Development 
of organic beings with remarkable skill and force. He argues 
from the analogy of domestic productions, from the changes which 
the embryos of many species undergo, from the difiiculty of dis- 
tinguishing species and varieties, and from the principle of general 
gradation, that species have been modified ; and he attributes the 
modification to the change of circumstances. The author (1855) 
has also treated Psychology on the principle of the necessary 
acquirement of each mental power and capacity by gradation. 

In 185^ M. Naudin, a distinguished botanist, expressly stated, 
in an admirable paper on the Origin of Species (' Revue Horticole, 
p. 102 ; since partly republished in the * Nouvelles Archives du 
Museum,' tom. i. p. 171), his belief that species are formed in 
an analogous manner as varieties are under cultivation ; and the 
latter process he attributes to man's power of selection. But he 
does not show how selection acts under nature. He believes, like 
Dean Herbert, that species, when nascent, were more plastic than at 
present. He lays weight on what he calls the principle of finality, 
** puissance myst^rieuse, indetermin^e ; fatalite pour les uns ; pour 
les autres, volonte providentielle, dont Taction incessante sur les 
§tres vivants determine, k toutes les ^poques de I'existence du 
monde, la forme, le volume, et la dur^e de chacun d'eux, en raison 



de sa destin^e dans Tordre de choscs dont il fait par tie. C'est cette 
puissance qui harmonise chaque membre k I'ensemble en I'appro- 
priant a la fonction qu'il doit remplir dans I'organisme general de 
la nature, fonction qui est pour lui sa raison d'etre." * 

In 1853 a celebrated geologist, Count Keyserling ('Bulletin do 
la Soc. Geolog.,' 2nd Ser., torn. x. p. 357), suggested that as new 
diseases, supposed to have been caused by some miasma, have 
arisen and spread over the world, so at certain periods the germs 
of existing species may have been chemically affected by circum- 
ambient molecules of a particular nature, and thus have given 
rise to new forms. 

In this same year, i853, Dr, Schaaffhausen published an ex- 
cellent pamphlet ('Yerhand. des Naturhist. Vereins der Preuss. 
Rheinlands,' &c.), in which he maintains the progressive develop- 
ment of organic forms on the earth. He infers that many species 
have kept true for long periods, whereas a few have become modi- 
fied. The distinction of species he explains by the destruction 
of intermediate graduated forms. " Thus living plants and animals 
are not separated from the extinct by new creations, but are to 
be regarded as their descendants through continued reproduction." 

A well-known French botanist, M. Lccoq, writes in 1854 
(' Etudes sur Olograph. Bot.,' tom. 1. p. 250), " On voit que nos 
recherches sur la fixite ou la variation de I'esp^ce, nous conduisent 
directement aux idees dmises, par deux hommes justement cel^bres, 
Geoffrey Saint-Hilaire et Goethe." Some other passages scattered 
through M. Lecoq's large work, make it a little doubtful how far he 
extends his views on the modification of species. 

The * Philosophy of Creation * has been treated in a masterly 
manner by the Kev. Baden Powell, in his * Essays on the Unity of 
Worlds,' 1855. Nothing can be more striking than the manner in 
which he shows that the introduction of new species is " a regular, 

* From references in Bronn's ' Untersuchuagen iiber die Entwiclceiungs- 
Gesetze,' it appears that the celebrated botanist and paleontologist Unger 
published, in 1852, his belief that species undergo development and modifi- 
cation. D'Alton, likewise, in Pander and Dalton's work on Fossil Sloths, 
expressed, in 1821, a similar belief. Similar views have, as is well known, 
been maintained by Oken in his mystical ' Natur-Philosophie.' From other 
references in Godron's work ' Sur I'Espfece,' it seems that Bory St. Vincent, 
Burdach, Poiret, and Fries, have all admitted that new species are continu- 
ally being produced. 

I may add, that of the thirty-four authors named in this Historical 
Sketch, who believe in the modification of species, or at least disbelieve in 
separate acts of creation, twenty-seven have written on special branches oj 
natural history or geology. 


not a casual phenomenon," or, as Sir John Herschel expresses it, 
*'a natural in contradistinction to a miraculous process." 

The thii'd volume of the ' Journal of the Linnean Society ' con- 
tains papers, read July 1st, 1858, by Mr. Wallace and myself, in 
which, as stated in the introductory remarks to this volume, the 
theory of Natural Selection is promulgated by Mr. Wallace with 
admirable force and clearness. 

Yon Baer, towards whom all zoologists feel so profound a respect, 
expressed about the year 1859 (sec Trof. Rudolph Wagner, * Zoolo- 
gisch-Anthropologische Untersuchungen,' 1861, s. 51) his convic- 
tion, chiefly grounded on the laws of geographical distribution, 
that forms now perfectly distinct have descended from a single 
parent- form. 

In June, 1859, Professor Huxley gave a lecture before the Eoyal 
Institution on the * Persistent Types of Animal life.' Referring to 
such cases, he remarks, " It is difficult to comprehend the meaning 
of such fiicts as these, if we suppose that each species of animal 
and plant J or each great type of organisation, was formed and 
placed upon the surface of the globe at long intervals by a distinct 
act of creative power; and it is well to recollect that such an 
assumption is as unsupported by tradition or revelation as it is 
opposed to the general analogy of nature. If, on the other hand, 
we view * Persistent Types ' in relation to that hypothesis which 
supposes the species living at any time to be the result of the 
gradual modification of pre-existing species — a hypothesis which, 
though unproven, and sadly damaged by some of its supporters, 
is yet the only one to which physiology lends any countenance ; 
their existence would seem to show that the amount of modification 
■,vhich living beings have undergone during geological time is but 
veiy small in relation to the whole series of changes which they 
have suffered." 

In December, 1859, Dr. Hooker published his ' Introduction to 
the Australian Flora.' In the first part of this great work he admits 
the truth of the descent and modification of species, and supports 
this doctrine by many original observations. 

The first edition of this work was published on November 24 th, 
1859, and the second edition on Januar}" 7tb, 18G0. 




When on board II.M.S. * Beagle,' as naturalist, I was mucli stnick 
with certain facts in the distribution of the organic beings inhabii 
ing South America, and in the geological relations of the present to 
the past inhabitants of that continent. These facts, as will be seen 
in the latter chapters of this volume, seemed to throw some light 
on the origin of species — that mystery of mysteries, as it has been 
called by one of our greatest philosophers. On my return home, it 
occurred to me, in 1837, that something might perhaps be made out 
on this question by patiently accumulating and reflecting on all 
BOx'-ts of facts which could possibly have any bearing on it. After 
five years' work I allowed myself to speculate on the subject, and 
drew up some short notes ; these I enlarged in 1844 into a sketch 
of the conclusions, wbich then seemed to me probable : from that 
period to the present day I have steadily pursued the same object. 
I hope that I may be excused for entering on these personal details, 
as I give them to show that I have not been hasty in coming to a 

My work is now (1859) nearly fmished ; but as it will take me 
many more years to complete it, and as my health .is far from- 
strong, I have been urged to publish this Abstract. I have more 
especially been induced to do this, as Mr. Wallace, who is now 
studying the natural history of the Malay archipelago, has arrived 
at almost exactly the same general conclusions that I have on the 
origin of species. In 1858 he sent me a memoir on this subject, 
with a request that I would forward it to Sir Charles Lyell, who 
sent it to the Linnean Society, and it is published in the third 
volume of the Journal of that Society. Sir C. Lyell and Dr. Hooker, 
who both knew of my work — the latter having read my sketch of 
1844 — honoured me by thinking it advisable to publish, with Mr. 
Wallace's excellent memoir, some brief extracts from my manu- 

This Abstract^ which I now publish, must necessarily be im- 
perfect. I cannot here give references and authorities for my 



several statements ; and I must trust to the reader reposing soms 
confidence in my accuracy. ISId doubt errors will have crept in, 
though I hope I have always been cautious in trusting to good 
iuthorities alone. I can here give only the general conclusions at 
which 1 have arrived, with a few facts in illustration, but which, 
I hope, in most cases will suffice. No one can feel more sensible 
i-han I do of the necessity of hereafter publishing in detail all the 
facts, with references, on which my conclusions have been grounded ; 
and I hope in a future work to do this. For I am well aware that 
scarcely a single point is discussed in this volume on which facts 
cannot be adduced, often apparently leading to conclusions directly 
opposite to those at which I have arrived. A fair result can be 
obtained only by fully stating and balancing the facts and argu- 
ments on both sides of each question ; and this is here impossible. 

I much regret that want of space prevents my having the satis- 
faction of acknowledging the generous assistance which I have 
received from very many naturalists, some of them personally un- 
known to me. I cannot, however, let this opportunity pass without 
expressing my deep obligations to Dr. Hooker, who, for the last 
fifteen years, has aided me in every possible \vslj by his large stores 
of knowledge and his excellent judgment. 

In considering the Origin of Species, it is quite conceivable that 
a naturalist, reflecting on the mutual affinities of organic beings, 
on their embryological relations, their geographical distribution, 
geological succession, and other such facts, might come to the con- 
clusion that species had not been independently created, but had 
descended, like varieties, from other species. Nevertheless, such a 
conclusion, even if well founded, would be unsatisfactory, until it 
could be shown how the innumerable species inhabiting this world 
have been modified, so as to acquire that perfection of structure 
and coadaptation which justly excites our admiration. Naturalists 
continually refer to external conditions, such as climate, food, &c., 
as the only possible cause of variation. In one limited sense, as 
we shall hereafter see, this may be true ; but it is preposterous to 
attribute to mere external conditions, the structure, for instance, of 
the woodpecker, with its feet, tail, beak, and tongue, so admirably 
adapted to catch insects under the bark of trees. In the case of the 
mistletoe, which draws its nourishment from certain trees, which 
has seeds that must be transported by certain birds, and which has 
flowers with separate sexes absolutely requiii^ig the agency of 
certain insects to biing pollen from one flower to the other, it is 
equally preposterous to account for the stricture of this parasite, 
wllh its relations to several distinct organic beings, by the offecti? 


of external conditions, or of habit, or of the volition of the plant 

It is, therefore, of the highest importance to gain a clear insight 
into the means of modification and coadaptatiou. At the commence- 
ment of my observations it seemed to me probable that a careful 
Htudy of domesticated animals and of cultivated plants would offer 
the best chance of making out this obscure problem. Nor have 
I been disappointed ; in this and in all other perplexing cases I 
have invariably found that our knowledge, imperfect though it be, ol 
variation under domestication, afforded the best and safest clue. I 
may venture to express my conviction of the high value of such 
studies, although they have been very commonly neglected by 

From these considerations, I shall devote the first chapter of this 
Abstract to Variation under Domestication. AVe shall thus see that 
a large amount of hereditary modification is at least possible ; and, 
what is equally or more important, v/e shall see how great is the 
power of man in accumulating by his Selection successive slight 
variations. I will then pass on to the variability of species in a 
state of nature ; but I shall, unfortunately, be compelled to treat 
this subject far too briefly, as it can be treated properly only by 
giving long catalogues of facts. We shall, however, be enabled to 
discuss what circumstances are most favourable to variation. In 
the next chapter the Struggle for Existence amongst all organic 
beings throughout the world, which inevitably follows from the 
high geometrical ratio of their increase, will be considered. This is 
the doctrine of Malthus, applied to the whole animal and vegetable 
kingdoms. As many more individuals of each species are born than 
can possibly survive; and as, consequently, there is a frequently 
recurring struggle for existence, it follows that any being, if it vary 
however slightly in any manner profitable to itself, under the com- 
plex and sometimes varying conditions of life, will have a better 
chance of surviving, and thus be naturally selected. From the 
strong principle of inheritance, any selected variety will tend to 
propagate its new and modified form. 

This fundamental subject of Natural Selection will be treated at 
some length in the fourth chapter; and we shall then see how 
Natural Selection almost inevitably causes much Extinction of the 
less improved forms of life, and leads to what I have called Diver- 
gence of Character. In the next chapter I shall discuss the complex 
and little known laws of variation. In the five succeeding chapters, 
the most apparent and gravest difficulties in accepting the theory 
will be given : namely, first, the difficulties of trarsitions, or how a 

B 2 


simple being or a simple organ can be changed and perfected ink* 
a highly developed being or into an elaborately constructed organ ; 
secondly, the subject of Instinct, or the mental powers of animals ; 
thirdly, Hybridism, or the infertility of species and the fertility of 
varieties when intercrossed ; and fourthly, the imperfection of the 
Geological Record. In the next chapter I shall consider the geo- 
logical succession of organic beings throughout time ; in the twelfth 
and thirteenth, their geographical distribution throughout space ; in 
the fourteenth, their classification or mutual affinities, both when 
mature and in an embryonic condition. In the last chapter I shall 
give a brief recapitulation of the whole work, and a few concluding 

No one ought to feel surprise at much remaining as yet un- 
explained in regard to the origin of species and varieties, if he make 
due allowance for our profound ignorance in regard to the mutual 
relations of the many beings which live around us. Who can ex- 
plain why one species ranges widely and is very numerous, and 
why another allied species has a narrow range and is rare ? Yet 
these relations are of the highest importance, for they determine the 
present welfare and, as I believe, the future success and modification 
of every inhabitant of this world. Still less do we know of the 
mutual relations of the innumerable inhabitants of the world during 
the many past geological epochs in its history. Although much 
remains obscure, and will long remain obscure, I can entertain no 
doubt, after the most deliberate study and dispassionate judgment 
of which I am capable, that the view which most naturalists until 
recently entertained, and which I formerly entertained — namely, 
that each species has been independently created — is erroneous. 
I am fully convinced that species are not immutable ; but that 
those belonging to what are called the same genera are lineal 
descendants of some other and generally extinct species, in the 
same manner as the acknowledged varieties of any one species are 
the descendants of that species Furthermore, I am convinced 
that Natural Selection has been the most important, but not the 
exclusive, means of modificatioD. 



Vabiation under Domestication. 

Causes of Variability — Effects of Habit and the use or disuse cf Parts — 
Correlated Variation — Inheritance — Character of Domestic Varieties 
— Difficulty of distinguishing between Varieties and Species — Origin 
of Domestic Varieties from one or more Species — Domestic Pigeons_ 
their Differences and Origin — Principles of Selection, anciently fol- 
lowed, their Effects — Methodical and Unconscious Selection — Un- 
known Origin of our Domestic Productions — Circumstances favour- 
able to Man's power of Selection. 

Causes of Variahillty. 

When we compare the individuals of the same variety or sub- 
variety of our older cultivated plants and animals, one of the first 
points which strikes us is, that they generally differ more from 
each other than do the individuals of any one species or variety in 
a state of nature. And if we reflect on the vast diversity of the 
plants and animals which have been cultivated, and which have 
varied during all ages under the most different climates and treat- 
ment, we are driven to conclude that this great variability is due 
to our domestic productions having been raised under conditions of 
life not so uniform as, and somewhat different from, those to which 
the parent-species had been exposed under nature. There is, also, 
some probability in the view propounded by Andrew Knight, that 
this variability may be partly connected with excess of food. It 
seems clear that organic beings must be exposed during several 
generations to new conditions to cause any great amount of varia- 
tion ; and that, when the organisation has once begun to vary, it 
generally continues varying for many generations. No case is on 
record of a variable organism ceasing to vary under cultivation. 
Our oldest cultivated plants, such as wheat, still yield new varie- 
ties: our oldest domesticated animals are still capable of rapid 
improvement or modification. 

As far as I am able to judge, after long attending to the subject; i 
the conditions of life appear to act in two ways, — directly on the/ 
whole organisation or on certain parts alone, and indirectly byl 
aifecting the reproductiye system. With respect to the direct 


action, we must bear in mind that in every case, as Professoi 
Weismann has lately insisted, and as I have incidentally shown in 
niv work on ' Variation under Domestication,' there are two 
factors : namely, the nature of the organism, and the nature of the 
conditions. The former seems to be much the more important • 
for nearly similar variations sometimes arise under, as far as wf 
can judge, dissimilar conditions ; and, on the other hand, dissimilar 
variations arise under conditions which appear to be nearly uniform. 
The effects on the offspring are either definite or indefinite. They 
may be considered as definite when all or nearly all the offspring of 
individuals exposed to certain conditions during several generations 
aro modified in the same manner. It is extremely difiicult to come 
to any conclusion in regard to the extent of the changes which have 
been thus definitely induced. There can, however, be little doubt 
about many slight changes, — such as size from the amount of food, ^ 
colour from the nature of the food, thickness of the skin and hair 
fi'om climate, &c. Each of the endless variations which we see in 
the plumage of our fowls must have had some efficient cause ; and | 
if the' same cause were to act uniformly during a long series of ' 
generations on many individuals, all probably would be modified in 
the same manner. Such facts as the complex and extraordinary 
out-growths which invariably follow from the insertion of a minute 
drop of poison by a gall-producing insect, show us what singular 
modifications might result in the case of plants from a chemical 
change in the nature of the sap. 

Indefinite variability is a much more common result of changed 
conditions than definite variability, and has probably played a more 
important part in the formation of our domestic races. We see 
indefinite variability in the endless slight peculiarities which dis- 
tinguish the individuals of the same species, and which cannot be 
accounted for by inheritance from either parent or from some more 
remote ancestor. Even strongly-marked differences occasionally 
appear in the young of the same litter, and in seedlings from the 
same seed-capsule. At long intervals of time, o-ut of millions ol 
individuals reared in the same country and fed on nearly the samo 
food, deviations of structure so strongly pronounced as to deserve 
to be called monstrosities arise ; but monstrosities cannot be 
separated by any distinct line from slighter variations. All such 
changes of structure, whether extremely slight or strongly marked, 
which appear amongst many individuals living together, may be 
considered as the indefinite effects of the conditions of life on each 
individual organism, in nearly the same manner as a chill aflfecTs 
different men in an indefinite manner, according to their stato 


of body or constitution, causing coughs or colds, rheumatism, or 
inflammations of various organs. 

With respect to what I have called the indirect action of changed 
conditions, namely, through the reproductive system being affected, 
WQ may infer that variability is thus induced, partly from the fact 
of this system being extremely sensitive to any change in the con- 
ditions, and partly from the similarity, as Kolreuter and others 
have remarked, between the variability which follows from the 
crossing of distinct species, and that which may be observed with 
plants and animals when reared under new or unnatural conditions. 
Many facts clearly show how eminently susceptible the reproduc- 
tive system is to very slight changes in the surrounding conditions. 
Nothing is more easy than to tame an animal, and few things more 
difficult than to get it to breed freely under confinement, even when 
the male and female unite. How many animals there are which 
will not breed, though kept in an almost free state in their native 
country ! This is generally, but erroneously, attributed to vitiated 
instincts. Many cultivated plants display the utmost vigour, and 
yet rarely or never seed ! In some few cases it has been discovered 
that a very trifling change, such as a little more or less water at 
some particular period of growth, will determine whether or not a 
plant will produce seeds. I cannot here give the details which 
I have collected and elsewhere published on this curious subject ; 
but to show how singular the laws are which determine the repro- 
duction of animals under confinement, I may mention that car- 
nivorous animals, even from the tropics, breed in this country 
pretty freely under confinement, with the exception of the planti- 
grades or bear family, which seldom produce j^oung ; whereaa 
carnivorous birds, with the rarest exceptions, hardly ever lay fertile 
eggs. Many exotic plants have pollen utterly worthless, in the 
same condition as in the most sterile hybrids. When, on the one 
hand, we see domesticated animals and plants, though often weak 
and sickly, breeding freely under confinement ; and when, on the 
other hand, we see individuals, though taken young from a state of 
nature joerfectly tamed, long-lived and healthy (of which I could 
give numerous instances), yet having their reproductive system so 
seriously affected by unperceived causes as to fail to act, we need 
not be surprised at this system, when it does act under confinement, 
acting irregularly, and producing offspring somewhat unlike their 
parents. I may add, that as some organisms breed freely under 
the most unnatural conditions (for instance, rabbits and ferrets kept 
in hutches), showing that their reproductive organs are not easily 
affected ; so will some animals and plants withstand domestication 


or cultivation, and vary very slightly — perhaps hardly more than in 
a state of nature. 

Some naturalists have maintained that all variations are con- 
nected with the act of sexual reproduction ; but this is certainly 
an error ; for I have given in another work a long list of " sporting 
plants," as they are called by gardeners ; — tliat is, of plants which 
have suddenly produced a single bud with a new and sometimes 
widely different character from that of the other buds on the same 
plant. These bud variations, as they may be named, can be pro- 
pagated by grafts, offsets, &c., and sometimes by seed. They occur 
rarely under nature, but are far from rare under culture. As a 
single bud out of the many thousands, produced year after year on 
the same tree under uniform conditions, has been known suddenly 
to assume a new character ; and as buds on distinct trees, growing 
under different conditions, have sometimes yielded nearly the same 
variety — for instance, buds on peach-trees producing nectarines, 
and buds on common roses producing moss-roses — we clearly see 
that the nature of the conditions is of subordinate importance in 
comparison with the nature of the organism in determining each 
particular form of variation ; — perhaps of not more importance than 
the nature of the spark, by which a mass of combustible matter is 
ignited, has in determining the nature of the flames. 

Effects of Habit and of the Use or Disuse of Farts; Coir elated 
Variation ; Inheritance, 

Changed habits produce an inherited effect, as in the period of the 
flowering of plants when transported fr©m one climate to another. 
With animals the increased use or disuse of parts has had a more 
marked influence ; thus I find in the domestic duck that the bones 
of the wing weigh less nnd the bones of the leg more, in proportion 
to the whole skeleton, than do the same bones in the wild-duck ; 
and this change may be safely attributed to the domestic duck 
flying much less, and walking more, than its wild parents. The 
great and inherited development of the udders in cows and goats in 
countries where they are habitually milked, in comparison with 
these organs in other countries, is probably another instance of the 
effects of use. Not one of our domestic animals can be named 
which has not in some country drooping ears ; and the view which 
has been suggested that the drooping is due to the disuse of the 
muscles of the ear, from the animals being seldom much alarmed, 
seems probable. 

Many laws regulate variation, some few of which can be dimly 


seen, and will hereafter be briefly discussed. I will here only 
allude to what may be called correlated variation. Important 
changes in the embryo or larva will probably entail changes in the 
mature animal. In monstrosities, the correlations between quite 
distinct parts are very curious ; and many instances are given in 
Isidore GeotTroy St. Ililaire's great work on this subject. Breeders 
believe that long limbs are almost always accompanied by an 
elongated head. Some instances of correlation are quite whimsical : 
thus cats which are entirely white and have blue eyes are generally 
deaf; but it has been lately stated by Mr. Tait that this is confined 
to the males. Colour and constitutional peculiarities go together, 
of which many remarkable cases could be given amongst animals 
and plants. From facts collected by Heusinger, it appears that 
white sheep and pigs are injured by certain plants, whilst dark- 
coloured individuals escape: Professor Wyman has recently com- 
municated to me a good illustration of this fact ; on asking some 
farmers in Virginia how it was that all their pigs were black, they 
informed him that the pigs ate the paint-root (Lachnanthes), 
which coloured their bones pink, and which caused the hoofs of all 
but the black varieties to drop off; and one of the "crackers" 
(i. e. Virginia squatters) added, '* we select the black members of a 
litter for raising, as they alone have a good chance of living," Hair- 
less dogs have imperfect teeth : long-haired and coarse-hairei 
animals are apt to have, as is asserted, long or many horns ; pigeons 
with feathered feet have skin between their outer toes ; pigeons 
with short beaks have small feet, and those with long beaks large 
feet. Hence if man goes on selecting, and thus augmenting, any 
pecuUarity, he will almost certainly modify unintentionally other 
parts of the structure, owing to the mysterious laws of correlation. 

The results of the various, unknown, or but dimly understood 
laws of variation are infinitely complex and diversified. It is weD 
worth while carefully to study the several treatises on some of 
our old cultivated plants, as on the hyacinth, potato, even the 
dahlia, &c. ; and it is really surprising to note the endless points of 
structure and constitution in which the varieties and sub-varieties 
difier slightly from each other. The whole organisation seems to 
have become plastic, and departs in a slight degree from that of the 
parental type. 

Any variation which is not inherited is unimportant for us. 
But the number and diversity of inheritable deviations of structure, 
both those of slight and those of considerable physiological impor- 
tance, are endless. Dr. Prosper Lucas's treatise, in two large 
TOlumes, is the fullest and the best on this subject. No breed ei 


doubts how strong is the tendency to inheritance ; that like pro* 
duces like is his fundamental belief: doubts have been thrown oa 
this principle only by theoretical writers. When any deviation ol 
structure often appears, and we see it in the father and child, we 
cannot tell whether it may not be due to the same cause having 
acted on both ; but when amongst individuals, apparently exposed 
to the same conditions, any very rare deviation, due to some 
extraordinary combination of circumstances, appears in the parent 
— say, once amongst several million individuals — and it reappears 
in the child, the mere doctrine of chances almost compels us to 
attribute its reappearance to inheritance. Every one must have 
heard of cases of albinism, prickly skin, hairy bodies, &c., appearing 
in several members of the same family. If strange and rare 
deviations of structure are really inherited, less strange and com- 
moner deviations may be freely admitted to be inheritable. 
Perhaps the correct way of viewing the whole subject would be, to 
look at the inheritance of every character whatever as the rule, and 
non-inheritance as the anomaly. 

The laws governing inheritance are for the most part unknown. 
No one can say why the same peculiarity in different individuals 
of the same species, or in different species, is sometimes inherited 
and sometimes not so ; why the child often reverts in certain 
characters to its grandfather or grandmother or more remote ances- 
tor ; why a peculiarity is often transmitted from one sex to both 
sexes, or to one sex alone, more commonly but not exclusively to 
the like sex. It is a fact of some importance to us, that peculiarities 
appearing in the males of our domestic breeds are often transmitted, 
either exclusively or in a much greater degree, to the males alone. 
A much more important rule, which I think may be trusted, is 
that, at whatever period of life a peculiarity first appears, it tends 
to re-appear in the offspring at a corresponding age, though some- 
times earlier. In many cases this could not be otherwise ; thus 
the inherited peculiarities in the horns of cattle could appear only 
in the offspring when nearly mature ; peculiarities in the silk- 
worm are known to appear at the corresponding caterpillar or 
cocoon stage. But hereditary diseases and some other facts make 
me believe that the rule has a wider extension, and that, when 
there is no apparent reason why a peculiarity should appear at 
any particular age, yet that it does tend to appear in the offspring 
at the same period at which it first appeared in the parent. I 
believe this rule to be of the highest importance in explaining the 
laws of embryology. These remarks are of course confined to the 
first appearance of the peculiarity, and not to the primary cause 


which may have acted on the ovules or on the male element ; in 
nearly the same manner as the increased length of the hoins in the 
cffspring from a short-horned cow by a long-horned bull, though 
appearing late in life, is clearly due to the male element. - 

Having alluded to the subject of reversion, I may here refer to a 
statement often made by naturalists — namely, that our domestic 
varieties, when run wild, gradually but invariably revert in charac- 
tor to their aboriginal stocks. Hence it has been argued that no 
deductions can be drawn from domestic races to species in a state of 
nature. I have in vain endeavoured to discover on what decisive 
facts the above statement has so often and so boldly been made. 
There would be great difficulty in proving its truth : we may safely 
conclude that very many of the most strongly marked domestic varie- 
ties could not possibly live in a wild state. In many cases we do not 
know what the aboriginal stock was, and so could not tell whether 
or not nearly perfect reversion had ensued. It would be necessary, 
in order to prevent the effects of intercrossing, that only a single 
variety should have been turned loose in its new home. Neverthe- 
less, as our varieties certainly do occasionally revert in some of 
their characters to ancestral forms, it seems to me not improbable 
that if we could succeed in naturalising, or were to cultivate, 
during many generations, the several races, for instance, of the 
cabbage, in very poor soil (in which case, however, some effect 
would have to be attributed to the definite action of the poor soil), 
that they would, to a large extent, or even wholly, revert to the 
wild aboriginal stock. Whether or not the experiment would 
succeed, is not of great importance for our line of argument ; for by 
the experiment itself the conditions of life are changed. If it could 
be shown that our domestic varieties manifested a strong tendency 
to reversion, — that is, to lose their acquired characters, whilst kept 
under the same conditions, and whilst kept in a considerable body, 
so that free intercrossing might check, by blending together, any 
slight deviations in their structure, in such case, I grant that we 
could deduce nothing from domestic varieties in regard to species. 
But there is not a shadow of evidence in favour of this view : to 
assert that we could not breed our cart and race-horses, long and 
Rhort-horned cattle, and poultry of various breeds, and esculent 
vegetables, for an unlimited number of generations, would be 
opposed to all experience. 


Character of Domestic Varieties: difficulty of distinguishing 
between Varieties and Species; origin of Domestic Varieties\ 
from one or more Species. ! 

When we look to the hereditary varieties or races of our domestic 
animals and plants, and compare them with closely allied species, 
we generally perceive in each domestic race, as already remarked, 1 
less uniformity of character than in true species. Domestic races ' 
often have a somewhat monstrous character ; by which I mean, 
that, although differing from each other, and from other species of 
the same genus, in several trifling respects, they often differ in an 
extreme degree in some one part, both when compared one with 
another, and more especially when compared with the species under 
nature to which they are nearest allied. With these exceptions 
(and with that of the perfect fertility of varieties when crossed, — a 
subject hereafter to be discussed), domestic races of the same species 
differ from each other in the same manner as do the closely-allied 
species of the same genus in a state of nature, but the differences 
in most cases are less in degree. This must be admitted as true, 
for the domestic races of many animals and plants have beec 
ranked by some competent judges as the descendants of aborigi- 
nally distinct species, and by other competent judges as mere 
varieties. If any well marked distinction existed between a 
domestic race and a species, this source of doubt would not so 
perpetually recur. It has often been stated that domebtic races 
do not differ from each other in characters of generic value. It can 
be shown that this statement is not correct ; but naturalists differ 
much in determining what characters are of generic value ; all 
such valuations being at present empirical. When it is explained 
now genera originate under nature, it will be seen that we have 
no right to expect often to find a generic amount of difference 
in our domesticated races. 

In attempting to estimate the amount of structural difference 
between allied dom.estic races, we are soon involved in doubt, 
from not knowing whether they are descended from one or several 
parent species. This point, if it could be cleared up, would be 
interesting ; if, for instance, it could be shown that the greyhound, 
bloodhou.nd, terrier, spaniel, and bull-dog, which we all know 
propagate their kind truly, were the offspring of any single species, 
then such facts would have great weight in making us doubt about 
the immutability of the many closely allied natural species — for 
instance, of the many foxes — inhabiting different quarters of the 
world. 1 do not believe, as we shall presently see, that the whole 


amount of difference between the several breeds of the do^ has bee?? 
produced under domestication ; I believe that a small part of the 
difference is due to tlieir being descended from distinct species. la 
the case of strongly marked races of some other domesticated 
species, there is presumptive or even strong evidence, that all are 
descended from a single wild stock. 

It has often been assumed that man has chosen for domestica- 
tion animals and plants having an extraordinary inherent tendency 
to vary, and likewise to withstand diverse climates I do not dispute 
that these capacities have added largely to the value of most of our 
domesticated productions ; but how could a savage possibly know, 
when he first tamed an animal, whether it would vary in succeeding 
generations, and whether it would endure other climates ? Has 
the little variability of the ass and goose, or the small power of 
endurance of warmth by the reindeer, or of cold by the common 
camel, prevented their domestication? I cannot doubt that if 
other animals and plants, equal in number to our domesticated 
productions, and belonging to equally diverse classes and countries, 
were taken from a state of nature, and could be made to breed for 
an equal number of generations under domestication, they would 
on an average vary as largely as the parent species of our existing 
domesticated productions have varied. 

In the case of most of our anciently domesticated animals and 
plants, it is not possible to come to any definite conclusion, whether 
they are descended fKom one or several wild species. The argument 
mainly relied on by those who believe in the multiple origin of our 
domestic animals is, that we find, in the most ancient times, on the 
monuments of Egypt, and in the lake-habitations of Switzerland, 
much diversity in the breeds; and that some of the?e ancient 
breeds closely resemble, or are even identical with, those still exist- 
ing. But this only throws far backwards the history of civilisation, 
and shows that animals were domesticated at a much earlier period 
than has hitherto been supposed. The lake-inhabitants of Swit- 
zerland cultivated several kinds of wheat and barley, the pea, the 
poppy for oil, and flax; and they possessed several domesticated 
animals. They also carried on commerce with other nations. All 
this clearly shows, as Heer has remarked, that they had at this 
early age progressed considerably in civilisation ; and this again 
implies a long continued previous period of less advanced civilisation, 
during which the domesticated animals, kept by different tribes in 
different districts, might have varied and given rise to distinct races. 
Since the discovery of flint tools in the superficial formations ol 
many parts of the world, all geologists believe that barbarian man 


existed at an enormously remote period ; and we know that at the 
present day there is hardly a tribe so barbarous, as not to have 
domesticated at least the dog. 
f> The origin of most of our domestic animals will probably for 
ever remain vague. But I may here state, that, looking to the 
domestic dogs of the whole world, I have, after a laborious collection 
of all known facts, come to the conclusion that several wild species 
of Canidee have been tamed, and that their blood, in some cases 
mingled together, flows in the veins of our domestic breeds. In 
regard to sheep and goats I can form no decided opinion. From 
facts communicated to me by Mr. Blyth, on the habits, voice, con- 
stitution, and structure of the humped Indian cattle, it is almos; 
certain that they are descended from a different aboriginal stock 
from our European cattle ; and some competent judges believe that 
these latter have had two or three wild progenitors, — whether or not 
these desei ve to be called species. This conclusion, as well as that 
of the specific distinction between the humped and common cattle, 
may, indeed, be looked upon as established by the admirable re- 
searches of Professor RUtimeyer. With respect to horses, from 
reasons which I cannot here give, I am doubtfully inclined to believe, 
in opposition to several authors, that all the races belong to the same 
species. Having kept nearly all the English breeds of the fowl 
alive, having bred and crossed them, and examined their skeletons, 
it appears to me almost certain that all are the descendants of the 
wild Indian fowl, Gallus bankiva ; and this is the conclusion of 
Mr. Blyth, and of others who have studied this bird in India. In 
regard to ducks and rabbits, some breeds of which differ much from 
each other, the evidence is clear that they are all descended 
from the common wild duck and rabbit. 

The doctrine of the origin of our several domestic races fron. 
several aboriginal stocks, has been carried to an absurd extreme bj 
some authors. They believe that every race which breeds true, lej 
the distinctive characters be ever so slight, has had its wild proto- 
type. At this rate there must have existed at least a score of 
species of wild cattle, as many sheep, and several goats, in Europe 
alone, and several even within Great Britain. One author believes 
that there formerly existed eleven wild species of sheep peculiar to 
Great Britain ! When we bear in mind that Britain has now not 
one peculiar mammal, and France but few distinct from those of 
Germany, and so with Hungary, Spain, &c., but that each of these 
kingdoms possesses several peculiar breeds of cattle, sheep, &c., we 
must admit that many domestic breeds must have originated in 
Europe ; f )r wh«3nce otherwise could they have been derived ? So it is 



ia India. Even in the case of the breeds of the domestic dog through- 
out the world, which I admit are descended from several wild spe- 
cies, it cannot be doubted that there has been an immense amount 
of inherited variation ; for who will believe that animals closely 
resembling the Italian greyhound, the bloodhound, the bull-dog, 
pug-dog, or Blenheim spaniel, &c. — so unlike all wild Canidte — 
ever existed in a state of nature ? It has often been loosely said 
that all our races of dogs have been produced by the crossing of a 
few aboriginal species ; but by crossing we can only get forms in 
some degree intermediate between their parents ; and if we account 
for our several domestic races by this process, we must admit the 
former existence of the most extreme forms, as the Italian grey- 
hound, bloodhound, bull-dog, &c., in the wild state. Moreover, 
the possibility of making distinct races by crossing has been greatly 
exaggerated. Many cases are on record, showing that a race may 
be modified by occasional crosses, if aided by the careful selection 
of the individuals which present the desired character; but to 
obtain a race intermediate between two quite distinct races, would 
be very difficult. Sir J. Sebright expressly experimented with this 
object, and failed. 1'he offspring from the first cross between two 
pure breeds is tolerably and sometimes (as I have found with 
pigeons) quite uniform in character, and everything seems simple 
enough ; but when these mongrels are crossed one with another for 
several generations, hardly two of them are alike, and then the 
difficulty of the task becomes manifest. 

Breeds of tJte Domestic Pigeon, their Differences and Origin. 

Believing that it is always best to study some special group, I 
have, after deliberation, taken up domestic pigeons. I have kept 
every breed which I could purchase or obtain, and have been most 
kindly favoured with skins from several quarters of the world, more 
especially by the Hon. W. Elliot from India, and by the Hon. C. 
Murray from Persia. Many treatises in different languages have 
been published on pigeons, and some of them are very important, 
as being of considerable antiquity. I have associated with several 
eminent fanciers, and have been permitted to join two of the London 
Pigeon Clubs. The diversity of the breeds is something astonishing. 
Compare the English carrier and the short-faced tumbler, and see 
the wonderful difference in their beaks, entailing corresponding 
differences in their skulls. The carrier, more especially the male 
bird, is also remarkable from the wonderful development of the 
caninculated skin about the head ; and this is accompanied by 
greatly elongated eyelids, very large external orifices to the' nostrila, 

36 DOMESTIC riGEOXS. [Chap. L 

and a wide gape of mouth. The short-faced tumbler has a beak in 
outline ahnost like that of a finch; and the common tumbler has 
the singular inherited habit of flying at a great height in a compact 
flock, and tumbling in the air head over heels. The runt is a bird 
of great size, with long massive beak and large feet ; some of the 
sub-breeds of runts have very long necks, others very long wings 
and tails, others singularly short tails. The barb is allied to the 
carrier, but, instead of a long beak, has a very short and broad one. 
The pouter has a much elongated body, wings, and legs ; and its 
enormously developed crop, which it glories in inflating, may well 
excite astonishment and even laughter. The turbit has a short and 
conical beak, with a line of reversed feathers down the breast ; and 
it has the habit of continually expanding, slightly, the upper part of 
the oesophagus. The Jacobin has the feathers so much reversed 
along the back of the neck that they form a hood ; and it has, pro- 
portionally to its size, elongated wing and tail feathers. The 
trumpeter and laugher, as their names express, utter a very difl'erent 
coo from the other breeds. The fantail has thirty or even forty 
tail-feathers, instead of twelve or fourteen — the normal number in 
all the members of the great pigeon family : these feathers are kept 
expanded, and are carried so erect, that in good birds the head and 
tail touch: the oil-gland is quite aborted. Several other less 
distinct breeds might be specified. 

In the skeletons of the several breeds, the development of the 
bones of the face in length and breadth and curvature differs enor- 
mously. The shape, as well as the breadth and length of the ramus 
of the lower jaw, varies in a highly remarkable manner. The 
caudal and sacral vertebra3 vary in number ; as does the number of 
ihe ribs, together with their relative breadth and the presence of 
processes. The size and shape of the apertures in the sternum are 
highly variable ; so is the degree of divergence and relative size of 
the two arms of the furcula. The proportional width of the gape 
of mouth, the proportional length of the eyelids, of the orifice of 
the nostrils, of the tongue (not always in strict correlation with the 
length of beak), the size of the crop and of the upper part of the 
oesophagus ; the development and abortion of the oil-gland ; the 
number of the primary wing and caudal feathers ; the relative 
length of the wing and tail to each other and to the body ; the 
relative length of the leg and foot ; the number of scutellaj on 
the toes, the development of skin between the toes, are all points 
of structure which are variable. The period at which the perfect 
plumage is acquired varies, as does the state of the down with wliich 
the nestling birds are clothed when hatched. The shape and size 


3f the eggs vary. The manner of flight, and in some breeds the 
Toice and disposition, differ remarkably. Lastly, in certain breeds, 
the males and females have come to differ in a slight degree from 
each other. 

Altogether at least a score of pigeons might be chosen, which, if 
bhown to an ornithologist, and he were told that they were wild 
birds, would certainly be ranked by him as well-defined species. 
Moreover, I do not believe that any ornithologist would in this 
case place the English carrier, the short-faced tumbler, the runt^ the 
barb, pouter, and fantail in the same genus ; more especially as in 
each of these breeds several truly-inherited sub-breeds, or species, as 
he would call them, could be shown him. 

Great as are the differences between the breeds of the pigeon, 
I am fully convinced that the common opinion of naturalists is 
correct, namely, that all are descended from the rock-pigeon 
(Columba livia), including under this term several geographical 
races or sub-species, which differ from each other in the most trifling 
respects. As several of the reasons which have led me to this 
belief are in some degree applicable in other case?, I will here briefly 
give them. If the several breeds are not varieties, and have not 
proceeded from the rock-pigeon, they must have descended from at 
least seven or eight aboriginal stocks ; for it is impossible to make 
the present domestic breeds by the crossing of any lesser number : 
how, for instance, could a pouter be produced by crossing two 
breeds unless one of the parent-stocks possessed the characteristic 
enormous crop? The supposed aboriginal stocks must all have 
been rock-pigeons, that is, they did not breed or willingly perch on 
trees. But besides C. livia, with its geographical sub-species, only 
two or three other species of rock-pigeons are known ; and these 
have not any of the characters of the domestic breeds. Hence the 
supposed aboriginal stocks must either still exist in the countries 
where they were originally domesticated, and yet be unknown to 
ornithologists ; and this, considering their size, habits, and remark- 
able characters, seems improbable ; or they must have become 
extinct in the wild state. But birds breeding on precipices, and 
good fliers, are unlikely to be exterminated ; and the common rock- 
pigeon, which has the same habits with the domestic breeds, has 
not been exterminated even on several of the smaller British islets, 
or on the shores of the Mediterranean. Hence the supposed exter- 
mination of so many species having similar habits with the rock- 
pigeon seems a very rash assumption. Morever, the several above- 
named domesticated breeds have been transported to all parts of the 
world, and, therefore, some of them must have been carried back 



18 DO.MESTIC PIGE0N3. [CiL^. 1. 

again into their native country ; but not one has become wild or 
feral, though the dovecot-pigeon, which is the rock-pigeon in a very 
slightly altered state, has become feral in several places. Again, 
all recent experience shows that it is difficult to get wild animals to 
breed freely under domestication; yet, on the hypothesis of thr 
multiple origin of our pigeons, it must be assumed that at least 
f^Y&i or eight species were so thoroughly domesticated in ancient 
times by half-civilised man, as to be quite prolific under con- 

An argument of great weight, and applicable in several other 
eases, is, that the above-specified breeds, though agreeing generally 
with the wild rock-pigeon in constitution, habits, voice, colouring, 
and in most parts of their structure, yet are certainly highly abnor- 
mal in other parts ; we may look in vain through the whole great 
family of Columbida3 for a beak like that of the English carrier, or 
that of the short-faced tumbler, or barb ; for reversed feathers like 
those of the Jacobin ; for a crop like that of the pouter ; for tail- 
feathers like those of the fantail. Hence it must be assumed not 
only that half-civilised man succeeded in thoroughly domesticating 
several species, but that he intentionally or by chance picked out 
extraordinarily abnormal species; and further, that these very 
species have since all become extinct or unknown. So many strange 
contingencies are improbable in the highest degree. 

Some facts in regard to the colouring of pigeons well deserve 
consideration. The rock-pigeon is of a slaty-blue, with white loins; 
but the Indian sub-species, 0. intermedia of Strickland, has this 
part bluish. The tail has a terminal dark bar, with the outer 
feathers externally edged at the base with white. The wings have 
two black bars. Some semi- domestic breeds, and some truly wild 
breeds, have, besides the two black bars, the wings chequered with 
black. These several marks do not occur together in any other 
species of the whole family. Now, in every one of the domestic 
breeds, taking thoroughly well-bred birds, all the above marks, even 
to the white edging of the outer tail-feathers, sometimes concur 
perfectly developed. Moreover, when birds belonging to two or 
more distinct breeds are crossed, none of which are blue or have 
any of the above-specified marks, the mongrel ofi'spring are very 
apt suddenly to acquire these characters, l^o give one instance out of 
several which I have observed : — I crossed some white fantails, which 
breed very true, with some black barbs — and it so happens that 
blue varieties of barbs are so rare that I never heard of an instance 
in England ; and the mongrels were black, brown, and mottled. I 
also crossed a barb with a spot, which is a white bird with a rod 


tail and red spot on the farehead, and which notoriously breeds very 
true ; the mongrels were dusky and mottled. 1 then crossed one of 
the mongrel barb-fantails with a mongrel barb-spot, and they pro- 
duced a bird of as beautiful a blue colour, with the white loins, 
double black wiug-bar, and barred and white-edged tail- feathers, as 
any wild rock-pigeon ! We can understand these facts, on the 
well-known principle of reversion to ancestral characters, if all 
the domestic breeds are descended from the rock-pigeon. But if 
we deny this, we must make one of the two following highly im- 
probable suppositions. Either, first, that all the several imagined 
aboriginal stocks were coloured and marked like the rock-pigeon, 
although no other existing species is thus coloured and marked, so 
that in each separate breed there might be a tendency to revert to 
the very same colours and markings. Or, secondly, that each 
breed, even the purest, has within a dozen, or at most within a 
score, of generations, been crossed by the rock-pigeon : 1 say within 
a dozen or twenty generations, for no instance is known of crossed 
descendants reverting to an ancestor of foreign blood, removed by a 
greater number of generations. In a breed which has been crossed 
only once, the tendency to revert to any character derived from 
such a cross will naturally become less and less, as in each succeed- 
ing generation there will be less of the foreign blood ; but when 
there has been no cross, and there is a tendency in the breed to 
revert to a character which was lost during some former generation, 
this tendency, for all that we can see to the contrary, may be 
transmitted undiminished for an indefinite number of generations. 
These two distinct cases of reversion are often confounded together 
by those who have written on inheritance. 

Lastly, the hybrids or mongrels from between all the breeds of 
. the pigeon are perfectly fertile, as I can state from my own obser- 
vations, purposely made, on the most distinct breeds. Now, hardly 
any cases have been ascertained with certainty of hybrids from two 
quite distinct species of animals being perfectly fertile. Some 
authors believe that long-continued domestication eliminates this 
strong tendency to sterility in species. From the history of the 
dog, and of some other domestic animals, this conclusion is pro- 
bably quite correct, if applied to species closely related to each 
other. But to extend it so far as to suppose that species, aborigi- 
. naUy as distinct as carriers, tumblers, pouters, and fantails now arCs 
should yield offspring perfectly fertile inter se, would be rash in the 

From these several reasons, namely, — the improbability of man 
haying formerly made seven or eight supposed species of pigeons tc 

c 2 


breed freely under domestication; — these supposed species "being 
quite unknown in a wild state, and their not having become any- 
where feral ; — these species presenting certain very abnormal cha- 
racters, as compared with all other Columbidse, though so like the 
rock-pigeon in most respects ; — the occasional re-appearance of 
the blue colour and various black marks in all the breeds, both 
when kept pure and when crossed ; — and lastly, the mongrel off- 
spring being perfectly fertile; — from these several reasons, taken 
together, we may safely conclude that all our domestic breeds are 
descended from the rock-pigeon or Columba livia with its geogra- 
Iphical s*ub-species. 

In favour of this view, I may add, firstly, that the wild C. livia 
has been found capable of domestication in Europe and in India; 
and that it agrees in habits and in a great number of points of struc- 
ture with all the domestic breeds. Secondly, that, although an 
English carrier or a short-faced tumbler differs immensely in certain 
characters from the rock-pigeon, yet that, hj comparing the several 
sub-breeds of these two races, more especially those brought from 
distant countries, we can make, between them and the rock-pigeon, 
an almost perfect series ; so we can in some other cases, but not 
with all the breeds. Thirdly, those characters whicli are mainly dis- 
tinctive of each breed are in each eminently variable, for instance 
the wattle and length of beak of the carrier, the shortness of that 
of the tumbler, and the number of tail-feathers in the fantail ; and 
the explanation of this fact will be obvious when we treat of Selec- 
tion. Fourthly, pigeons have been watched and tended with the 
utmost care, and loved by many people. They have been domesti- 
cated for thousands of years in several quarters of the world ; the 
eorliest known record of pigeons is in the fifth ^Egyptian dynasty, 
about 3000 b.c, as was pointed out to me by Professor Lepsius ; 
but Mr. Birch informs me that pigeons are given in a bill of fare in 
the previous dynasty. In the time of the Komans, as we hear from 
Pliny, immense prices were given for pigeons ; " nay, they are come 
to this pass, that they can reckon up their pedigree and race." 
Pigeons were much valued by Akber Khan in India, about the year 
1600 ; never less than 20,000 pigeons were taken with the court. 
" The monarchs of Iran and Turan sent him some very rare birds ; ** 
and, continues the courtly historian, " His Majesty by crossing the 
breeds, which method was never practised before, has improved them 
astonishingly." About this same period the Dutch were as eager 
about pigeons as were the old Romans. The paramount importance 
of these considerations in explaining the immense amount of vari- 
ation which pigeons have undergone, will likewise be obvious when 


WB treat of Selection. We shall then, also, see how it is that tho 
several breeds so often have a somewhat monstrous character. It 
is also a most favourable circumstance for the production of dis- 
tinct breeds, that male and female pigeons can be easily mated for 
life; and thus different breeds can be kept together in the same 
aviary. '^ 

I have discussed the probable origin of domestic pigeons at some, 
yet quite insufficient, length ; because when I first kept pigeons 
and watched the several kinds, well knowing how truly they breed, 
I felt fully as much difficulty in believing that since they had been 
domesticated they had all proceeded from a common parent, as any 
naturalist could in coming to a similar conclusion in regard to the 
many species of finches, or other groups of birds, in nature. One 
circumstance has struck me much ; namely, that nearly all the 
breeders of the various domestic animals and the cultivators of 
plants, with whom I have conversed, or whose treatises I have read, 
are firmly convinced that the several breeds to which each has at- 
tended, are descended from so many aboriginally distinct species. 
Ask, as I have asked, a celebrated raiser of Hereford cattle, whether 
his cattle might not have descended from Long-horns, or both from 
a common parent-stock, and he will laugh you to scorn. 1 have 
never met a pigeon, or poultry, or duck, or rabbit fancier, who was 
not fully convinced that each main breed was descended from a dis- 
tinct species. Van Mens, in his treatise on pears and apples, shows 
how utterly he disbelieves that the several sorts, for instance a 
Kibston-pippin or Codlin-apple, could ever have proceeded from the 
seeds of the same tree. Innumerable other examples could be given. 
The explanation, I think, is simple: from long-continued study 
they are strongly impressed with the differences between the several 
races ; and though they well know that each race varies slightly, 
for they win their prizes by selecting such slight differences, yet 
they ignore all general arguments, and refuse to sum up in their 
minds slight differences accumulated during many successive gene- 
rations. May not those naturalists who, knowing far less of the 
laws of inheritance than does the breeder, and knowing no more 
than he does of the intermediate links in the long lines of descent, 
yet admit that many of our domestic races are descended from the 
same parents — may they not learn a lesson of caution, when they 
deride the idea of species in a state of nature being lineal descendants 
of other species ? 


Frincijyles of Selection anciently folloived, and their Effects. 

Let us now briefly consider the steps by which domestic races 
have been produced, either from one or from several allied species. 
Some effect may be attributed to the direct and definite action of 
the external conditions of life, and some to habit; but he would bo 
a bold man who would account by such agencies for the differences 
between a dray and race horse, a greyhound and bloodhound, a car- 
rier and tumbler pigeon. One of the most remarkable features in 
our domesticated races is that we see in them adaptation, not indeed 
to the animal's or plant's own good, but to man's use or fancy, 
Some variations useful to him have probably arisen suddenly, or by 
one step ; many botanists, for instance, believe that the fuller's teasel, 
with its hooks, which cannot be rivalled by any mechanical con- 
trivance, is only a variety of the wild Dipsacus ; and this amount 
of change may have suddenly arisen in a seedling. So it has pro- 
bably been with the turnspit dog ; and this is known to have been 
the case with the ancon sheep. But when we compare the dray- 
horse and race-horse, the dromedary and camel, the various breeds 
of sheep fitted either for cultivated land or mountain pasture, with 
the wool of one breed good for one purpose, and that of another 
breed for another purpose ; when we compare the many breeds of 
dogs, each good for man in different ways ; when we compare the 
game-cock, so pertinacious in battle, with other breeds so little quar- 
i-elsome, with "everlasting layers" which never desire to sit, and 
with the bantam so small and elegant ; when we compare the host 
of agricultural, culinary, orchard, and flower-garden races of plants, 
most useful to man at different seasons and for different purposes, or so 
beautiful in his eyes, we must, I think, look further than to mere 
variability. We cannot suppose that all the breeds were suddenly 
produced as perfect and as useful as we now see them ; indeed, in 
many cases, we know that this has not been their history. The 
key is man's power of accumulative selection : nature gives succes- 
sive variations ; man adds them up in certain directions useful to 
him. In this sense he may be said to have made for himself useful 

The great power of this principle of selection is not hypothetical. 
It is certain that several of our eminent breeders have, even within 
a single lifetime, modified to a large extent their breeds of cattle and 
sheep. In order fully to realise what they have done, it is almost 
necessary to read several of the many treatises devoted to this sub- 
ject, and to inspect the animals. Bleeders habitually speak of an 
animal's organisation as something plastic, which they can mode? 

Chaf. l] selection by man. 23 

aliQOflt as they please. If I had space I could quote numerous x>a8- 
sagcs to this effect from highly competent authorities. Youatt^ 
who was probably better acquainted with the works of agriculturists 
than almost any other individual, and who was himself a very good 
judge of animals, speaks of the principle of selection as " that which 
enables the agriculturist, not only to modify the character of his 
flock, but to change it altogether. It is the magician's wand, by 
means of which he may summon into life whatever form and mould 
he pleases." Lord Somerville, speaking of what breeders have dont 
for sheep, says : — " It would seem as if they had chalked out upon 
a wall a form perfect in itself, and then had given it existence." In 
Saxony the importance of the principle of selection in regard to 
merino sheep is so fully recognised, that men follow it as a trade : 
the sheep are placed on a table and are studied, like a picture by a 
connoisseur ; this is done three times at intervals of months, and the 
sheep are each time marked and classed, so that the very best may 
ultimately be selected for breeding. 

What English breeders have actually effected is proved by the 
enormous prices given for animals with a good pedigree ; and these 
have been exported to almost every quarter of the world. The 
improvement is by no means generally due to crossing different 
breeds ; all the best breeders are strongly opposed to this practice, 
except sometimes amongst closely allied sub-breeds. And when a 
cross has been made, the closest selection is far m.ore indispensable 
even than in ordinary cases. If selection consisted merely in sepa- 
rating some very distinct variety, and breeding from it, the principle 
would be so obvious as hardly to be worth notice ; but its import- 
ance consists in the great effect produced by the accumulation in 
one direction, during successive generations, of differences absolutely 
inappreciable by an uneducated eye — differences which I for one 
have vainly attempted to appreciate. Not one man in a thousand 
has accuracy of eye and judgment sufficient to become an eminent 
breeder. If gifted with these qualities, and he studies his subject 
for years, and devotes his lifetime to it with indomitable perse- 
verance, he will succeed, and may make great improvements ; if he 
wants any of these qualities, he will assuredly fail. Few would 
readily believe in the natural capacity and years of practice requisite 
to become even a skilful pigeon-fancier. 

The same principles are followed by horticulturists ; but the vari- 
ations are here often more abrupt. No one suyjposes that our 
choicest productions have been produced by a single variation from 
the aboriginal stock. We have proofs that this has not been so in 
eeveral cases in which exact records have been kept; thuu, to gJTe a 



very trifling instance, the steadily-increasing size of the common 
gooseberry may be quoted. We see an astonishing improvement in 
many florists' flowers, when the flowers of the present day are com« 
pared with drawings made only twenty or thirty years ago. When 
ft race of plants is once pretty well established, the seed-raisers do 
not pick out the best plants, but merely go over their seed-bods, 
and pull up the "rogues," as they call the plants that deviate 
from the proper standard. With animals this kind of selection is, 
in fact, likewise followed ; for hardly any one is so careless as to 
breed from his worst animals. 

In regard to plants, there is another means of observing the 
accumulated effects of selection — namely, by comparing the diver- 
sity of flowers in the different varieties of the same species in the 
flower-garden ; the diversity of leaves, pods, or tubers, or whatever 
part is valued, in the kitchen-garden, in comparison with the 
flowers of the same varieties ; and the diversity of fruit of the same 
species in the orchard, in comparison with the leaves and flowers 
of the same set of varieties. See how different the leaves of the 
cabbage are, and how extremely alike the flowers ; how unlike the 
flowers of the heartsease are, and how alike the leaves; how much 
the fruit of the different kinds of gooseberries differ in size, colour, 
shape, and hairiness, and yet the flowers present very slight diffe- 
rences. It is not that the varieties which differ largely in some one 
point do not differ at all in other points ; this is hardly ever, — I 
speak after careful observation, — perhaps never, the case. The law 
of correlated variation, the importance of which should never be 
overlooked, will ensure some differences ; but, as a general rule, it 
cannot be doubted that the continued selection of slight variations, 
either in the leaves, the flowers, or the fruit, will produce races dif- 
fering from each other chiefly in these characters. 

It may be objected that the principle of selection has been 
reduced to methodical practice for scarcely more than three-quarters 
of a century ; it has certainly been more attended to of late years, 
and many treatises have been published on the subject ; and the 
result has been, in a corresponding degree, rapid and important. 
But it is very far from true that the principle is a modern discovery. 
I could give several references to works of high antiquity, in which 
the full importance of the principle is acknowledged. In rude and 
barbarous periods of English history choice animals were cften im- 
ported, and laws were passed to prevent their exportation : the 
destruction of horses under a certain size was ordered, and this may 
be compared to the " roguing " of plants by nurserymen. The prin- 
ciple of selection I find distinctly given in an ancient Chinese encj- 

^ 1 ^ / 


clopsedia. Explicit rules are laid down by some of the Eoman 
classical writers. From passages in Genesis, it is clear that the 
colour of domestic animals was at that early period attended to. 
Savages now sometimes cross their dogs with wild canine animals, 
to improve the breed, and they formerly did so, as is attested by 
passages in Pliny. The savages in South Africa match their 
draught cattle by colour, as do some of the Esquimaux their teams 
of dogs. Livingstone states that good domestic breeds are highly 
valued by the negroes in the interior of Africa who have not associ- 
ated with Europeans. Some of these facts do not show actual 
selection, but they show that the breeding of domestic animals was 
carefully attended to in ancient times, and is now attended to by 
the lowest savages. It would, indeed, have been a strange fact, had 
attention not been paid to breeding, for the inheritance of good and 
bad qualities is so obvious. 

Unconscious Selection, 

At the present time, eminent breeders try by methodical selection, 
with a distinct object in view, to make a new strain or sub-breed, 
superior to anything of the kind in the country. But, for our pur- 
pose, a form of Selection, which may be called Unconscious, and 
which results from every one trying to possess and breed from the 
best individual animals, is more important. Thus, a man who 
intends keeping pointers naturally tries to get as good dogs as he 
can, and afterwards breeds from his own best dogs, but he has no 
wish or expectation of permanently altering the breed. Neverthe- 
less we may infer that this process, continued during centuries, 
would improve and modify any breed, in the same way as Bake- 
well, Collins, &c., by this very same process, only carried on more 
methodically, did greatly modify, even during their lifetimes, the 
forms and qualities of their cattle. Slow and insensible changes of 
this kind can never be recognised unless actual measurements or 
careful drawings of the breeds in question have been made long ago^ 
which may serve for comparison. In some cases, however, un- 
changed, or but little changed individuals of the same breed exist 
in less civilised districts, where the breed has been less improved. 
There is reason to believe that King Charles's spaniel has been un- 
consciously modified to a large extent since the time of that monarch. 
Some highly competent authorities are convinced that the setter is 
directly derived from the spaniel, and has probably been slowly 
altered from it. It is known that the English pointer has beeu 
gi'eatly changed within the last century, and in this case the chauge 
has, it is believed, been chiefly effected by crosses with the foxhound : 


but what concerns us is, that the change has been effected uncon* 
Eciously and gradually, and yet so effectually, that, though the old 
Spanish pointer certainly came from Spain, Mr. Borrow has not seen, 
as I am informed by him, any native dog in Spain like our pointer. 

By a similar process of selection, and by careful training, English 
racehorses have come to surpass in fleetness and size the parent 
Arabs, so that the latter, by the regulations for the Goodwood Races, 
are favoured in the weights which they carry. Lord Spencer and 
others have shown how the cattle of England have increased in 
weight and in early maturity, compared with the stock formerly 
kept in this country. By comparing the accounts given in various 
old treatises of the former and present state of carrier and tumbler 
pigeons in Britain, India, and Persia, we can trace the stages through 
which they have insensibly passed, and come to differ so greatly 
from the rock-pigeon. 

Youatt gives an excellent illustration of the effects of a course of 
selection, which may be considered as unconscious, in so far that the 
breeders could never have expected, or even wished, to produce 
the result which ensued — namely, the production of two distinct 
strains. The two flocks of Leicester sheep kept by Mr. Buckley 
and Mr. Burgess, as Mr. Youatt remarks, " have been purely bred 
from the original stock of Mr. Bakewell for upwards of fifty years. 
There is not a suspicion existing in the mind of any one at all 
acquainted with the subject, that the owner of either of them has 
deviated in any one instance from the pure blood of Mr. Bakewell's 
flock, and yet the difference between the sheep possessed by these 
two gentlemen is so great that they have the apjiearance of being 
quite different varieties." 

If there exist savages so barbarous as never to think of the inherited 
character of the offspring of their domestic animals, yet any one 
animal particularly useful to them, for any special purpose, would 
be carefully preserved during famines and other accidents, to which 
savages are so liable, and such choice animals would thus generally 
leave more offspring than the inferior ones ; so that in this case 
there would be a kind of unconscious selection going on. We see 
the \ alue set on animals even by the barbarians of Tierra del Fuego,^ 
by their killing and devouring their old women, in times of dearth, 
us of less value than their dogs. 

In plants the same gradual process of improvement, through the 
occasional preservation of the best individuals, whether or not 
sufficiently distinct to be ranked at their first appearance as distinct 
varieties, and whether or not two or more species or races have 
become blended together by crossing, may plainly be recognised in 


the increased size and beauty which we now see in the varieties of 
the heartsease, rose, pelargonium, dahlia, and other plants, when 
compared with the older varieties or with their parent-stocks. No 
one would ever expect to get a first ->rate hearsease or dahlia from 
the seed of a wild plant. No one would expect to raise a first-rate 
melting pear from the seed of the wild pear, though he might 
succeed from a poor seedling growing wild, if it had come from a 
garden-stock. The pear, though cultivated in classical times, 
appears, from Pliny's description, to have been a fruit of very 
inferior quality. I have seen great surprise expressed in horti- 
cultural works at the wonderful skill of gardeners, in having pro- 
duced such splendid results from such poor materials ; but the art 
has been simple, and, as far as the final result is concerned, has 
been followed almost unconsciously. It has consisted in always 
cultivating the best known variety, sowing its seeds, and, when a 
slightly better variety chanced to appear, selecting it, and so on- 
wards. But the gardeners cf the classical period, who cultivated 
the best pears which they could procure, never thought what 
splendid fruit we should eat ; though we owe our excellent fruit, 
in some small degree, to their having naturally chosen and preserved 
the best varieties they could anywhere find. 

A large amount of change, thus slowly and unconsciously ac- 
cumulated, explains, as I believe, the well-known fact, that in a 
number of cases we cannot recognise, and therefore do not know, 
the wild parent-stocks of the plants which have been longest culti- 
vated in our flower and kitchen gardens. If it has taken centuries 
or thousands of years to improve or modify most of our plants up to 
their present standard of usefulness to man, we can understand how 
it is that neither Australia, the Cape of Good Hope, nor any other 
region inhabited by quite uncivilised man, has afforded us a single 
plant worth culture. It is not that these countries, so rich in species, 
do not by a strange chance possess the aboriginal stocks of any use- 
ful plants, but that the native plants have not been improved by 
continued selection up to a standard of perfection comparable with 
that acquired by the plants in countries anciently civilised. 

In regard to the domestic animals kept by uncivilised man, it 
should not be overlooked that they almost always have to struggle 
for their own food, at least during certain seasons. And in two 
countries very differently circumstanced, individuals of the sarue 
species, having slightly different constitutions or stnicture, would 
orten succeed better in the one country than in the other ; and thus 
by a process of " natural selection," as will hereafter be more fully 
explained, two sub-breeds might be formed. This, f>erhap8, partly 


explains v:hj the varieties kept by savages, as has been remarked 
by some authors, have more of the character of true species than the 
varieties kept in civilised countries. 

On the view here given of the important part which selection by 
man has played, it becomes at once obvious, how it is that our 
domestic races show adaptation in their structure or in their habits 
to man's wants or fancies. We can, I think, further understand the 
frequently abnormal character of our domestic races, and likewise 
their differences being so great in external characters, and relatively 
so slight in internal parts or organs. Man can hardly select, or 
only with much difficulty, any deviation of structure excepting such 
as is externally visible ; and indeed he rarely cares for what is 
internal. He can never act by selection, excepting on variations 
which are first given to him in some slight degree by nature. No 
man would ever try to make a fantail till he saw a pigeon with 
a tail developed in some slight degree in an unusual manner, or a 
pouter till he saw a pigeon w^ith a crop of somewhat unusual size; 
and the more abnormal or unusual any character was when it first 
appeared, the more likely it would be to catch his attention. But 
to use such an expression as trying to make a fantail, is, I have no 
doubt, in most cases, utterly incorrect. The man who first selected 
a pigeon with a slightly larger tail, never dreamed what the descend • 
ants of that pigeon would become through long-continued, partly? 
unconscious and partly methodical, selection. Perhaps the parent- 
bird of all fantails had only fourteen tail-feathers somewhat expanded, 
like the present Java fantail, or like individuals of other and distinct 
breeds, in which as many as seventeen tail-feathers have been 
counted. Perhaps the first pouter-pigeon did not inflate its crop 
much more than the turbit now does the upper part of its oeso- 
phagus, — a habit which is disregarded by all fanciers, as it is not 
one of the points of the breed. 

Nor let it be thought that some great deviation of structure 
would be necessary to catch the fancier's eye : he perceives ex- 
tremely small differences, and it is in human nature to value slyij 
novelty, however slight, in one's own possession. Nor must the 
value which would formerly have been set on any slight difierences 
in the individuals cf the same species, be judged of by the value 
which is now set on them, after several breeds have fairly been 
established. It is known that with pigeons many slight variations 
now occasionally appear, but these are rejected as faults or devia- 
tions from the standard of perfection in each breed. The common 
goose has not given rise to any marked varieties ; hence the Tou- 
louse and the common breed, which differ only in colour, that most 




fleeting of characters, have lately been exhibited as distinct at our 

These views appear to explain what has sometimes been noticed 
— namely, that we know hardly anything about the origin or history 
of any of our domestic breeds. But, in fact, a breed, like a dialect 
of a language, can hardly be said to have a distinct origin. A man 
preserves and breeds from an individual with some slight deviation 
of structure, or takes more care than usual in matching his best 
animals, and thus improves them, and the improved animals slowly 
spread in the immediate neighbourhood. But they will as yet 
hardly have a distinct name, and from being only slightly valued, 
their history will ha;re been disregarded. When further improved 
by the same slow and gradual process, they will spread more widely, 
and will be recognised as something distinct and valuable, and will 
then probably first receive a provincial name. In semi-civilised 
countries, with little free communication, the spreading of a new 
sub-breed would be a slow process. As soon as the points of value 
are once acknowledged, the principle, as I have called it, of un- 
conscious selection will always tend, — perhaps more at one period 
than at another, as the breed rises or falls in fashion, — perhaps more 
in one district than in another, according to the state of civilisation 
of the inhabitants, — slowly to add to the characteristic features of 
the breed, whatever they may be. But the chance will be infinitely 
small of any record having been preserved of such slow, varying, 
and insensible changes. 

Circumstances favourable to Man's Power of Selection. 

I will now say a few words on the circumstances, favourable, or 
the reverse, to man's power of selection. A high degree of vari- 
ability is obviously favourable, as freely giving the materials for 
selection to work on ; not that mere individual differences are not 
amply sufficient, with extreme care, to allow of the accumulation 
of a large amount of modification in almost any desired direction. 
But as variations manifestly useful or pleasing to man appear 
only occasionally, the chance of their appearance will be much 
increased by a large number of individuals being kept. Hence, 
number is of the highest importance for success. On this principle 
Marshall formerly remarked, with respect to the sheep of parts of 
Yorkshire, " as they generally belong to poor people, and are mostly 
in small lots, they never can be improved." On the other hand, 
nurserymen, from keeping large stocks of the same plant, are gener- 
ally far more successful than amateurs in raising new and valuable 
varieties A large number of individuals of an animal or plant car 


be reared only wtere the conditions for its propagation are favour- 
able, When the individuals are scanty, all will be allovrcd to breed, 
whatever their quality may be, and this will effectually prevent 
selection. But probably the most important element is that the 
animal or plant should be so highly valued by man, that the closest 
attention is paid to even the slightest deviations in its qiLalities or 
structure. Unless such attention be paid nothing can be effected, 
I have seen it gravely remarked, that it was most fortunate that 
the strawberry began to vary just when gardeners began to attend 
to this plant. No doubt the strawberry had always varied since 
it was cultivated, but the slight varieties had been neglected. As 
soon, however, as gardeners picked out individual plants with 
slightly larger, earlier, or better fruit, and raised seedlings from 
them, and again picked out the best seedlings and bred from them, 
then (with some aid by crossing distinct species) those many 
admirable varieties of the strawberry were raised which have ap- 
peared during the last half-century. 

With animals, facility in preventing crosses is an important 
element in the formation of new races, — at least, in a country 
which is already stocked with other races. In this respect en- 
closure of the land plays a part. Wandering savages or the in- 
habitants of open plains rarely possess more than one breed of the 
same species. Pigeons can be mated for life, and this is a great 
^convenience to the fancier, for thus many races may be improved 
and kept true, though mingled in the same aviary ; and this cir- 
cumstance must have largely favoured the formation of new breeds. 
Pigeons, I may add, can be propagated in great numbers and at a 
very quick rate, and inferior birds may be freely rejected, as when 
killed they serve for food. On the other hand, cats, from their 
nocturnal rambling habits, cannot be easily matched, and, although 
so much valued by women and children, we rarely see a distinct 
breed long kept up ; such breeds as we do sometimes see are almost 
always imported from some other country. Although I do not 
doubt that some domestic animals vary less than others, yet the 
rarity or absence of distinct breeds of the cat, the donkey, peacock, 
goose, (fee, may be attributed in main part to selection not having 
been brought into play : in cats, from the difficulty in pairing them ; 
in donkeys, from only a few being kept by poor people, and little 
attention paid to their breeding ; for recently in certain parts of 
Spain and of the United States this animal has been surprisingly 
modified and improved by careful selection : in peacocks, from not 
being very easily reared and a large stock not kept : in geese, from 
bsing valuable only for two purposes, food and feathers, and more 


especially from no pleasure having been felt in the display of distinct 
breeds ; but the goose, under the conditions to which it is exposed 
\ihen domesticated, seems to have a singularly inflexible organisa- 
tion, though it has varied to a slight extent, as I have elsewhere 

Some authors have maintained that the amount of variation in 
our domestic productions is soon reached, and can never afterwards 
be exceeded. It would be somewhat rash to assert that the limit 
has been attained in any one case ; for almost all our animals and 
plants have been greatly improved in many ways within a recent 
period ; and this implies variation. It would be equally rash to 
assert that characters now increased to their utmost limit, could 
not, after remaining fixed for many centuries, again vary under 
new conditions of life. No doubt, as Mr. Wallace has remarked 
with much truth, a limit will be at last reached. For instance, 
there must be a limit to the fleetness of any terrestrial animal, as 
this will be determined by the friction to be overcome, the weight 
of body to be carried, and the powxr of contraction in the muscular 
fibres. But what concerns us is that the domestic varieties of the 
same species differ from each other in almost eveiy character, which 
man has attended to and selected, more than do the distinct species 
of the same genera. Isidore Geoffrey St. Hilaire has proved this in 
regard to size, and so it is with colour and probably with the length 
of hair. With respect to fleetness, which depends on many bodily 
characters, Eclipse was far fleeter, and a dray-horse is incomparably 
S'fcronger than any two natural species belonging to the same 
genus. So with plants, the seeds of the different varieties of the 
bean or maize probably differ more in size, than do the seeds of 
the distinct species in any one genus in the same two families. 
The same remark holds good in regard to the fruit of the several 
varieties of the plum, and still more strongly with the melon, as 
well as in many other analogous cases. 

To sum up on the origin of our domestic races of animals and 
plants. Changed conditions of life are of the highest importance in 
causing variability, both by acting directlj'' on the organisation, and 
indirectly by affecting the reproductive system. It is not probable 
that variability is an inherent and necessary contingent, under all 
circumstances. The greater or less force of inheritance and rever- 
sion determine whether variations shall endure. Variability is 
governed by many unknown laws, of which correlated growth is 
probably the most important. Something, but how much we do 
net know, may be attributed to the definite action of the conditions 
?f life. Some, perhaps a great, efi'ect may be attributed to the 


Increased use or disnse of parts. The final result is thus rendered 
infinitely com])lex. In some cases the intercrossing of aboriginally 
distinct species appears to have played an important part in the 
origin of our breeds. When several breeds have once been formed 
in any country, their occasional intercrossing, with the aid of selec^ 
tion, has, no doubt, largely aided in the formation of new sub- 
breeds ; but the importance of crossing has been much exaggerated, 
both in regard to animals and to those plants which are pro- 
pagated by seed. With plants which are temporarily propagated 
by cuttings, buds, &c., the importance of crossing is immense ; for 
the cultivator may here disregard the extreme variability both of 
hybrids and of mongrels, and the sterility of hybrids ; but plants 
Qot propagated by seed are of little importance to us, for their 
endurance is only temporary. Over all these causes of Change, the 
accumulative action of Selection, whether applied methodically and 
quickly, or unconsciously and slowly but more efficiently, scorns to 
have been the predominant rov\'er. 



Yaiiiation under Nature. 

Variability — Individual differences — Doubtful species — Wide ranging, 
much diffused, and common species, vary most — Species of the larger 
genera in each country vary more frequently than the species of the 
smaller genera — Many of the species of the larger genera i*esemble 
varieties in being very closely, but unequally, related to each other, 
and in having restricted ranges. 

Before applying the principles arrived at in the last chapter to 
organic beings in a state of nature, we must briefly discuss whether 
these latter are subject to any variation. To treat this subject 
properly, a long catalogue of dry facts ought to be given ; but these 
I shall reserve for a future work. Nor shall I here discuss the 
-various definitions which have been given of the term species. No 
one definition has satisfied all naturalists; yet every naturalist 
knows vaguely what he means when he speaks of a species. 
Generally the term includes the unknown element of a distinct 
act of creation. The term " variety " is almost equally difficult 
to define; but here community of descent is almost universally 
implied, though it can rarely be proved. We have also what 
are called monstrosities ; but they graduate into varieties. By 
a monstrosity I presume is meant some considerable deviation of 
structure, generally injurious, or not useful to the species. Some 
authors use the term " variation " in a technical sense, as implying 
a modification directly due to the physical conditions of life ; and 
" variations " in this sense are supposed not to be inherited ; but 
who can say that the dwarfed condition of shells in the brackish 
waters of the Baltic, or dwarfed plants on Alpine summits, or the 
thicker fur of an animal from far northwards, would not in some 
cases be inherited for at least a few generations ? and in this case I 
presume that the form would be called a variety. 

It may be doubted whether sudden and considerable deviations 
«f structure such as we occasionally see in our domestic productions, 
more especially with plants, are ever pennanently propagated in a 
fitate of nature. Almost every part of every organic being is so 
beautifully related to its complex conditions of life that it seems as 



impro'bable that any part should have been suddenly produced 
perfect, as that a complex machine should have been invented by 
man in a perfect state. Under domestication monstrosities some- 
times occur which resemble normal structures in widely diiTerent 
animals. Thus pigs have occasionally been born with a sort of 
proboscis, and if any wild species of the same genus had naturally 
possessed a proboscis, it might have been argued that this had 
appeared as a monstrosity ; but I have as yet failed to find, after 
diligent search, cases of monstrosities resembling normal structures 
in nearly allied forms, and these alone bear on the question. If 
monstrous forms of this kind ever do appear in a state of nature and 
are capable of reproduction (which is not always the case), as they 
occur rarely and singly, their preservation would depend on 
unusually favourable circumstances. They would, also, during the 
first and succeeding generations cross with the ordinary form, and 
thus their abnormal character would almost inevitably be lost. 
But I shall have to return in a future chapter to the preservation 
and perpetuation of single or occasional variations. 

Individual Differences. 

The many slight differences which appear in the offspring from 
the same parents, or which it may be presumed have thus arisen, 
from being observed in the individuals of the same species in- 
habiting the same confined locality, may be called individual 
differences. No one supposes that all the individuals of the same 
species are cast in the same actual mould. These individual 
differences are of the highest importance for us, for they are often 
inherited, as must be familiar to every one ; and they thus afford 
materials for natural selection to act on and accumulate, in the 
same manner as man accumulates in any given direction individual 
differences in his domesticated productions. These individual 
differences generally affect what naturalists consider unimportant 
parts ; but I could show by a long catalogue of facts, that parts 
which must be called important, whether viewed under a physio- 
logical or classificatory point of view, sometimes vary in the 
individuals of the same species. I am convinced that the most 
experienced naturalist would be surprised at the number of the 
cases of variability, even in important parts of structure, which ho 
could collect on good authority, as I have collected, during a course 
of years. It should be remembered that systematiSts are far from 
being pleased at finding variability in important characters, and 
that there are not many men who will laboriously examine internal 
and important organs, and compare them in many specimens of 


the same species. It would never have been expected that the 
branching of the main nerves close to the great central ganglion of 
an insect would have been variable in the same species ; it migbt 
have been thought that changes of this nature could have been 
effected only by slow degrees ; yet Sir J. Lubbock has shown a 
degree of variability in these main nerves in Coccus, which may 
almost be compared to the irregular branching of the stem of a tree. 
This philosophical naturalist, I may add, has also shown that the 
muscles in the larvse of certain insects are far from uniform. 
Authors sometimes argue in a circle when they state that important 
organs never vary ; for these same authors practically rank those 
parts as important (as some few naturalists have honestly confessed) 
which do not vary ; and, under this point of view, no instance will 
ever be found of an important part varying ; but under any other 
point of view many instances assuredly can be given. 

There is one point connected with individual differences, which is 
extremely perplexing : I refer to those genera which have been called 
" protean " or " polymorphic," in which the species present an inor- 
dinate amount of variation. With respect to many of these forms, 
hardly two naturalists agree whether to rank them as species or as 
varieties. We may instance Rubus, Rosa, and Hieracium amongst 
plants, several genera of insects and of Brachiopod shells. In most 
polymorphic genera some of the species have fixed and definite 
characters. Genera which are polymorphic in one country seem to 
be, with a few exceptions, polymorphic in other countries, and like- 
wise, judging from Brachiopod shells, at former periods of time. 
These facts are very perplexing, for they seem to show that this 
kind of variability is independent of the conditions of life. I am 
inclined to suspect that we see, at least in some of these polymorphio 
genera, variations which are of no service or disservice to the species, 
and which consequently have not been seized on and rendered definite 
by natural selection, as hereafter to be explained. 

Individuals of the same species often present, as is known to 
every one, great differences of structure, independently of variation, 
as in the two sexes of various animals, in the two or three castes of 
sterile females or workers amongst insects, and in the immature and 
larval states of many of the lower animals. There are, also, cases 
of dimorphism and trimorphism, both with animals and plants. 
Thus, Mr. Wallace, who has lately called attention to the subject, 
has shown that the females of certain species of butterflies, in the 
Malayan archipelago, regularly appear under two or even three 
conspicuously distinct forms, not connected by intermediate varieties. 
"Fritz Miiller has described analagous but more extraordinary cases 

D 2 


with the males of certain Brazilir.n Crustaceans : thus, the male 
of a Tanais regularly occurs under two distinct forms; one of 
these has strong and differently shaped pincers, and the other has 
antennae much more abundantly furnished with smelling-hairs. 
Although in most of these cases, the two or three forms, both 
with animals and plants, are not now connected by intermediate 
gi'adations, it is probable that they were once thus connected. 
Mr. Wallace, for instance, describes a certain butterfly which pre- 
sents in the same island a great range of varieties connected by 
intermediate links, and the extreme links of the chain closely 
resemble the two forms of an allied dimorphic species inhabiting 
another part of the Malay archipelago. Thus also with ants, the 
several worker-castes are generally quite distinct ; but in some cases, 
as we shall hereafter see, the castes are connected together by finely 
graduated varieties. So it is, as I have myself observed, with some 
dimorphic plants. It certainly at first appears a highly remarkable 
fact that the same female butterfly should have the power of pro- 
ducing at the same time three distinct female fonns and a male ; 
and that an hermaphrodite plant should produce from the same 
seed-capsule three distinct hermaphrodite forms, bearing three 
different kinds of females and three or even six different kinds 
of males. Nevertheless these cases are only exaggerations of the 
common fact that the female produces offspring of two sexes which 
sometimes differ from each other in a wonderful manner. 

Doubtful Species. 

The forms which possess in some considerable degree the cha- 
racter of species, but which are so closely similar to other forms, oi 
are so closely linked to them by intermediate gradations, that 
naturalists do not like to rank them as distinct species, are in 
several respects the most important for us. We have every 
reason to believe that many of these doubtful and closely allied 
forms have permanently retained their characters for a long 
time ; for as long, as far as we know, as have good and true 
species. Practically, when a naturalist can unite by means of 
intermediate links any two forms, he treats the one as a variety 
of the other ; ranking the most common, but sometimes the one 
first described, as the species, and the other as the variety. 
But cases of great difficulty, which I will not here enumerate, 
sometimes arise in deciding whether or not to rank one form as a 
variety of another, even when they are closely connected by inter- 
mediate links ; nor will the commonly-assumed hybrid nature cf 


tlie intermediate forms always remove the difficulty. In yery many 
cases, however, one form is ranked as a variety of another, not 
because the intermediate links have actually been found, but 
because analogy leads the observer to suppose either that they dc 
now somewhere exist, or may formerly have existed ; and here a 
wide door for the entry of doubt and conjecture is opened. 

Hence, in determining whether a form should be ranked as al 
species or a variety, the opinion of naturalists having sound judg- 
ment and wide experience seems the only guide to follow. We 
must, however, in many cases, decide by a majority of naturalists, 
for few well-marked and well-known varieties can be named whicli 
have not been ranked as species by at least some competent) 

That varieties of this doubtful nature are far from uncommon 
cannot be disputed. Compare the several floras of Great Britain, of 
France, or of the United States, drawn up by different botanists^ 
and see what a surprising number of forms have been ranked by 
one botanist as good species, and by another as mere varieties. 
JVIr. H. C. Watson, to whom I lie under deep obligation for assistance 
of all kinds, has marked for me 182 British plants, which arc 
generally considered as varieties, but which have all been ranked 
by botanists as species; and in making this list he has omitted 
many trifling varieties, but which nevertheless have been ranked 
by some botanists as species, and he has entirely omitted several 
highly polymorphic genera. Under genera, including the most 
polymorphic forms, Mr. Babington gives 251 species, whereas 
Mr. Bentham gives only 112, — a difference of 139 doubtful forms ! 
Amongst animals which unite for each birth, and which are highly 
locomotive, doubtful forms, ranked by one zoologist as a species and 
by another as a variety, can rarely be found within the same 
country, but are common in separated areas. How many of the 
birds and insects in North America and Europe, which differ 
very slightly from each other, have been ranked by one eminent 
naturalist as undoubted species, and by another as varieties, or, as 
they are often called, geographical races ! Mr. Wallace, in several 
valuable papers on the various animals, especially on the Lepi- 
doptera, inhabiting the islands of the great Malayan archipelago, 
shows that they may be classed under four heads, namely, as vari- 
able forms, as local forms, as geographical races or sub-species, and 
as true representative species. The first or variable forms vary 
much within the limits of the same island. The local forms 
are moderately constant and distinct in each separate island ; but 
when all from the several islands are compared together, the dif- 


ferences are seen to be so slight and graduated, that- it is impossible 
to define or describe them, though at the samo time the extreme 
forms are sufficiently distinct. The geographical races or sub-species 
are local forms completely fixed and isolated ; but as they do not 
differ from each other by strongly marked and important characters, 
*' there is no possible test but individual opinion to determine 
which of them shall be considered as species and which as varieties.*' 
Lastly, representative species fill the same place in the natural 
economy of each island as do the local forms and sub-species ; but 
as they are distinguished from each other by a greater amount of 
difference than that between the local forms and sub-species, they 
are almost universally ranked by naturalists as true species. Never- 
theless, no certain criterion can possibly be given by which variable 
forms, local forms, sub-species, and representative species can be 

Many years ago, when comparing, and seeing others compare, the 
birds from, the closely neighbouring islands of the Galapagos archi- 
pelago, one with another, and with those from the American main- 
land, I was much struck how entirely vague and arbitrary is the 
distinction between species and varieties. On the islets of the 
little Madeira group there are many insects which are charac- 
terized as varieties in Mr. Wollaston's admirable work, but 
which would certainly be ranked as distinct species by many 
entomologists. Even Ireland has a few animals, now generally 
regarded as varieties, but which have been ranked as species 
by some zoologists. Several experienced ornithologists consider 
our British red grouse as only a strongly-marked race of a 
Norwegian species, whereas the greater number rank it as an 
undoubted species peculiar to Great Britain. A wide distance 
between the homes of two doubtful forms leads many naturalists to 
rank them as distinct species ; but what distance, it has been well 
asked, will suffice ; if that between America and Europe is ample, 
will that between Europe and the Azores, or Madeira, or the 
Canaries, or between the several islets of these small archipelagos, 
be sufficient ? 

Mr. B. D. Walsh, a distinguished entomologist of the United 
States, has described what he calls Phytophagic varieties and Phy- 
tophagic species. Most vegetable-feeding insects live on one kind 
of plant or on one group of plants ; some feed indiscriminately on 
many kinds, but do not in consequence vary. In several cases, 
however, insects found living on different plants, have been observed 
by Mr. Walsh to present in their larval or mature state, or in both 
states, slig:ht, though constant differences in colour, size, or in the 


nature of tlieir secretions. In some instances tlie males alone, in 
ether instances both males and females, have been observed thus 
10 differ in a slight degree. When the differences are rather more 
strongly marked, and when both sexes and all ages are affected, thflr*^ 
forms are ranked by all entomologists as good species. But no 
observer can determine for another, even if he can do so for himself, 
wliich of these Phytophagic forms ought to be called species and 
which varieties. Mr. Walsh ranks the forms which it may be 
supposed would freely intercross, as varieties; and those which 
appear to have lost this power, as species. As the differences depend 
on the insects having long fed on distinct plants, it cannot be 
expected that intermediate links connecting the several forms should 
now be found. The naturalist thus loses his best guide in deter- 
mining whether to rank doubtful forms as varieties or species. This 
likewise necessarily occurs with closely allied organisms, which 
inhabit distinct continents or islands. When, on the other hand, 
an animal or plant ranges over the same continent, or inhabits many 
islands in the same archipelago, and presents different forms in the 
different areas, thers is always a good chance that intermediate 
foims will be discovered which will link together the extreme 
states ; and these are then degraded to the rank of varieties. 

Some few naturalists maintain that animals never present varie- 
ties ; but then these same naturalists rank the slightest difference 
as of specific value ; and when the same identical form is met with 
in two distant countries, or in two geological formations, they 
believe that two distinct species are hidden under the same dress. 
The term species thus comes to be a mere useless abstraction, im- 
plying and assuming a separate act of creation. It is certain that 
many forms, considered by highly-competent judges to be varieties, 
resemble species so completely in character, that they have been thus 
ranked by other highly-competent judges. But to discuss whether 
they ought to be called species or varieties, before any definition of 
these terms has been generally accepted, is vainly to beat the air. 

Many of the cases of strongly-marked varieties or doubtful species 
well deserve consideration ; for several interesting lines of argument, 
from geographical distribution, analogical variation, hybridism, &c., 
have been brought to bear in the attempt to determine their rank ; 
but space does not here permit me to discuss them. Close investi- 
gation, in many cases, will no doubt bring naturalists to agree how 
to rank doubtful forms. Yet it must be confessed that it is in the 
host known countries that we find the greatest number of them. 
I have been struck with the fact, that if any animal or plant in a 
state of nature be highly useful to man, or from any cause closely 


attracts his attention, varieties of it will alii.ost universally be 
found recorded. These varieties, moreover, will often be ranked by 
some authors as species. Look at the common oak, how closely it 
has been studied ; yet a German author makes more than a dozen 
species out of forms, which are almost universally considered by 
other botanists to be varieties ; and in this country the highest 
botanical authorities and practical men can be quoted to show that 
the sessile and pedunculated oaks are either good and distinct species 
or mere varieties. 

I may here allude to a remarkable memoir lately published by 
A. de CandoUe, on the oaks of the whole world. No one ever had 
more ample materials for the discrimination of the species, or could 
have worked on them with more zeal and sagacity. He first gives 
in detail all the many points of structure which vary in the several 
species, and estimates numerically the relative frequency of the 
variations. He specifies above a dozen characters which may be 
found varying even on the same branch, sometimes according to 
age or development, sometimes without any assignable reason. 
Such characters are not of course of specific value, but they are, as 
Asa Gray has remarked in commenting on this memoir, such as 
generally enter into specific definitions. De Candolle then goes on 
to say that he gives the rank of species to the forms that differ by 
characters never varying on the same tree, and never found con- 
nected by intermediate states. After this discussion, the result of 
so much labour, he emphatically remarks : " They are mistaken, 
who repeat that the greater part of our species are clearly limited, 
and that the doubtful species are in a feeble minority. This seemed 
to be true, so long as a genus was imperfectly known, and its species 
were founded upon a few specimens, that is to say, were provisionaL 
Just as we come to know them better, intermediate forms flow in, 
and doubts as to specific limits augment." He also adds that it is 
the best known species which present the greatest number of spon- 
taneous varieties and sub- varieties. Thus Quercus robur has twenty- 
3ight varieties, all of which, excepting six, are clustered round three 
sub-species, namely, Q. pedunculata, sessiliflora, and pubescens. 
The forms which connect these three sub-species are comparatively 
rare ; and, as Asa Gray again remarks, if these connecting forms, 
which are now rare, were to become wholly extinct, the three sub- 
species would hold exactly the same relation to each other, as do 
the four or five provisionally admitted species which closely sur- 
round the typical Quercus robur. Finally, De Candolle admits 
that out of the 300 species, which will be et.umerated in his Pro- 
dromus as belonging to the oak family, at least two-thirds are 


provisional species, that is, arc not known strictly to fulfil the defi- 
nition above given of a true species. It should he added that De 
CandoUe no longer believes that species are immutable creations, but 
concludes that the derivative theory is the most natural one, " and 
the most accordant with the known facts in palaaontology, geo- 
graphical botany and zoology, of anatomical structure and classifi- 

When a young naturalist commences the study of a group of 
organisms quite imknown to him, he is at first much perplexed in 
determining what differences to consider as specific, and what as 
varietal ; for he knows nothing of the amount and kind of variation 
to which the group is subject ; and this shows, at least, how very 
generally there is some variation. But if he confine his attention 
to one class within one country, he will soon make up his mind 
how to rank most of the doubtful forms. His general tendency 
will be to make many species, for he will become impressed, just 
like the pigeon or poultry fancier before alluded to, with the amount 
of difference in the forms which he is continually studying ; and 
he has little general knowledge of analogical variation in other 
groups and in other countries, by which to correct his first impres- 
sions. As he extends the range of his observations, he will meet 
with more cases of difficulty; for he will encounter a greater 
number of closely-allied forms. But if his observations be widely 
extended, he will in the end generally be able to make up his own 
mind ; but he will succeed in this at the expense of admitting much 
variation, — and the truth of this admission will often be disputed 
by other naturalists. When he comes to study allied forms brought 
from countries not now continuous, in which case he cannot hope 
to find intermediate links, he will be compelled to trust almost 
entirely to analogy, and his difficulties will rise to a climax. 

Certainly no clear line of demarcation has as yet been drawn 
between species and sub-species — that is, the forms which in the 
opinion of some naturalists come very near to, but do not quite 
arrive at, the rank of species : or, again, between sub-species and 
well-marked varieties, or between lesser varieties and individual dif- 
ferences. These differences blend into each other by an insensible 
series ; and a series impresses the mind with the idea of an actual 

Hence I look at individual differences, though of small interest to 
the systematist, as of the highest importance for us, as being the 
first steps towards such slight varieties as are barely thought worth 
recording in works on natural history. And I look at varieties 
which arc in any degree more distinct and permanent, as stepa 


towards more strongly-marked and permanent varieties ; and at tlie 
latter, as leading to sub-species, and then to species. The passage 
from one stage of difference to another may, in manji cascy, be the 
simple result of the nature of the organism and of the different 
physical conditions to which it has long been exposed ; but with 
respect to the more important and adaptive characters, the passage 
from one stage of difference to another, may be safely attributed to 
the cumulative action of natural selection, hereafter to be explained, 
and to the effects of the increased use or disuse of parts. A well- 
marked variety may therefore be called an incipient species ; but 
whether this belief is justifiable must be judged by the weight 
of the various facts and considerations to be given througnout this 

It need not be supposed that all varieties or incipient species 
attain the rank of species. They may become extinct, or they may 
endure as varieties for very long periods, as has been shown to be 
the case by Mr. Wollaston with the varieties of certain fossil land- 
shells in Madeira, and with plants by Gaston de Saporta. If a 
variety were to flourish so as to exceed in numbers the parent 
species, it would them rank as the species, and the species as tho 
variety ; or it might come to supplant and exterminate the parent 
species; or both might co-exist, and both rank as independent 
species. But we shall hereafter return to this subject. 

From these remarks it will be seen that I look at the term species 
as one arbitrarily given, for the sake of convenience, to a set of 
individuals closely resembling each other, and that it does not 
essentially differ from the term variety, which is given to less 
distinct and more fluctuating forms. The term variety, again, in 
comparison with mere individual differences, is also applied arbi- 
trarily, for convenience' sake. 

Wide-ranging J muchrdiffusedj and common Species vary most. 

Guided by theoretical considerations, I thought that some in- 
teresting results might be obtained in regard to the nature and 
relations of the species which vary most, by tabulating all the 
varieties in several well-worked floras. At first this seemed a 
simple task ; but Mr. H. C. Watson, to whom I am much indebted 
for valuable advice and assistance on this subject, soon convinced 
me that there were many difficulties, as did subsequently Dr 
Hooker, even in stronger terms. I shall reserve for a future work 
the discussion of these difficulties, and the tables of the proportional 
numbers of the varying species. Dr. Hooker permits me to add, 
that after having carefully read my manuscript, and examined the 


tables, lie thinks that the following statements are fairly well esta- 
blished. The whole subject, however, treated as it necessarily here 
is with much brevity, is rather perplexing, and allusions cannot bo 
avoided to the " f truggle for existence," " divergence of character," 
and other questions, hereafter to be discussed. 

Alphonse de CandoUe and others have shown that plants which 
have very wide ranges generally present varieties ; and this might 
have been expected, as they are exposed to diverse physical condi- 
tions, and as they come into competition (which, ss we shall here- 
after see, is an equally or more important circumstance) with 
different sets of organic beings. But my tables further show that, 
in any limited country, the species which are the most common, 
that is abound most in individuals, and the species which are most 
widely diffused within their own country (and this is a different 
consideration from wide range, and to a certain extent from com- 
monness), oftenest give rise to varieties sufficiently well-marked 
to have been recorded in botanical works. Hence it is the most 
flourishing, or, as they may be called, the dominant species, — those 
which range widely, are the most diffused in their own country, and 
are the most numerous in individuals, — which oftenest produce 
well-marked varieties, or, as I consider them, incipient species. And 
this, perhaps, might have been anticipated ,■ for, as varieties, in 
order to become in any degree permanent, necessarily have to 
struggle with the other inhabitants of the country, the species which 
are already dominant will be the most likely to yield offspring, 
which, though in some slight degree modified, still inherit those 
advantages that enabled their parents to become dominant over 
their compatriots. In these remarks on predominance, it should be 
understood that reference is made only to the forms which come 
into competition with each other, and more especially to the mem- 
bers of the same genus or class having nearly similar habits of life. 
With respect to the number of individuals or commonness ol 
species, the comparison of course relates only to the members 
of the same group. One of the higher plants may be said to be 
dominant if it be more numerous in individuals and more widely 
diffused than the other plants of the same country, which live under 
Dearly the same conditions. A plant of this kind is not the less 
dominant because some conferva inhabiting the water or some 
parasitic fungus is infinitely more numerous in individuals, and more 
widely diffused. But if the conferva or parasitic fungus exceeds its 
alUes in the above respects, it will then be dominant within its own 


Spscies of the Larger Genera in each Country vary more frequenUy 
than the Species of the Smaller Genera. 

If the plants inhabiting a country, as described in any Flora, be 
divided into two equal masses, all those in the larger genera (i.e., 
those including many species) being placed on one side, and all 
those in the smaller genera on the other side, the former will bo 
foujid to include a somewhat larger number of the very common and 
much diffused or dominant species. This might have been antici- 
pated ; for the mere fact of many species of the same genus inhabit- 
ing any country, shows that there is something in the organic or 
inorganic conditions of that country favourable to the genus ; and, 
consequently, we might have expected to have found in the larger 
genera, or those including many species, a larger proportional number 
of dominant species. But so many causes tend to obscure this 
result, that I am surprised that my tables show even a small majority 
on the side of the larger genera. I will here allude to only two 
causes of obscurity. Fresh-water and salt-loving plants generally 
have very wide ranges and are much diffused, but this seems to be 
connected with the nature of the stations inhabited by them, and 
has little or no relation to the size of the genera to which the 
species belong. Again, plants low in the scale of organisation are 
generally much more widely diffused than plants higher in the scale j 
and here again there is no close relation to the size of the genera. 
The cause of lowly-organised plants ranging widely will be discussed 
in our chapter on Geographical Distribution. 

From looking at species as only strongly-marked and well-defined 
varieties, I was led to anticipate that the species of the larger genera 
in each country would oftener present varieties, than the species of 
the smaller genera ; for wherever many closely related species (i.e., 
species of the same genus) have been formed, many varieties or 
incipient species ought, as a general rule, to be now forming. 
Where many large trees grow, we expect to find saplings. Whero 
many species of a genus have been formed through variation, cir- 
cumstances have been favourable for variation ; and hence we might 
expect that the circumstances would generally be still favourable 
to variation. On the other hand, if we look at each species as a 
special act of creation, there is no apparent reason why more 
varieties should occur in a group having many species, than in one 
having lew. 

To test the truth of this anticipation I have arranged the plants 
of twelve countries, and the coleopterous insects of two districts, into 
two nearly equal masses, the species of the larger genera on one 


side, and those of the smaller genera on the other side, and it has 
invariably proved to be the case that a larger proportion of the 
species on the side of the larger genera presented varieties, than on 
the side of the smaller genera. Moreover, the species of the large 
genera which present any varieties, invariably present a larger 
average number of varieties than do the species of the small genera. 
Both these results follow when another division is made, and when 
all the least genera, with from only one to four species, are altogether 
excluded from the tables. These facts are of plain signification on 
the view that species are only strongly-marked and permanent 
varieties ; for wherever many species of the same genus have been 
formed, or where, if we may use the expression, the manufactory of 
species has been active, we ought generally to find the manufactory 
still in action, more especially as we have every reason to believe 
the process of manufacturing new species to be a slow one. And 
this certainly holds true, if varieties be looked at as incipient species ; 
for my tables clearly show as a general rule that, wherever many 
species of a genus have been formed, the species of that genus 
present a number of varieties, that is of incipient species, beyond 
the average. It is not that all large genera are now varying much, 
and are thus increasing in the number of their species, or that no 
small genera are now varying and increasing ; for if this had been 
so, it would have been fatal to my theory ; inasmuch as geology 
plainly tells us that small genera have in the lapse of time often 
increased greatly in size ; and that large genera ?iave often come to 
their maxima, declined, and disappeared. All that we want to show 
is, that, where many species of a genus have been formed, on an 
average many are still forming ; and this certainly holds good. 

Many of the Species induded within the Larger Genera resemble 
^ Varieties in heing very closely, but unequally, related to each 
■ other, and in having restricted ranges. 

There are other relations between the species of large genera and 
their recorded varieties which deserve notice. We have seen that 
there is no infallible criterion by which to distinguish species and 
well-marked varieties ; and when intermediate links have not been 
found between doubtful forms, naturalists arc compelled to come to 
a determination by the amount of difference between them, judging 
\)y analogy whether or not the amount sufiBces to raise one or both 
to the rank of species. Hence the amount of difference is one very 
important criterion in settling whether two forms should be ranked 
as species or varieties. Now Fries has remarked in regard to plants, 
and Westwood in regard to insects, that in large genera the amount 


of difference between the species is often exceedingly small. I have 
endeavoured to test this numerically by averages, and, as far as my 
imperfect results go, they coRfirm the view. I have also consulted 
some sagacious and experienced observers, and, after deliberation^ 
they concur in this view. In this respect, therefore, the species 
of the larger genera resemble varieties, more than do the species oi 
the smaller genera. Or the case may be put in another way, and it 
may be said, that in the larger genera, in which a number of varie- 
ties or incipient species greater than the average are now manufac- 
turing, many of the species already manufactured still to a certain 
extent resemble varieties, for they differ from each other by less 
than the usual amount of difference. 

Moreover, the species of the larger genera are related to each 
other, in the same manner as the varieties of any one species are 
related to each other. No naturalist pretends that all the species of 
a genus are equally distinct from each other ; they may generally 
be divided into sub-genera, or sections, or lesser groups. As Fries 
has well remarked, little groups of species are generally clustered 
like satellites around other species. And what are varieties but 
groups of forms, unequally related to each other, and clustered round 
certain forms — that is, round their parent-species. Undoubtedly 
there is one most important point of difference between varieties 
and species ; namely, that the amount of difference between varie- 
ties, when compared with each other or with their parent-species, is 
much less than that between the species of the same genus, liut whcji 
we come to discuss the principle, as I call it, of Divergence of 
Character, we shall see how this may be explained, and how the 
lesser differences between varieties tend to increase into the greater 
differences between species. 

There is one other point which is worth notice. Varieties gene- 
rally have much restricted ranges ; this statement is indeed 
scarcely more than a truism, for, if a variety were found to have a 
wider range than that of its supposed parent-species, their denomi- 
nations would be reversed. But there is reason to believe that the 
species which are very closely allied to other species, and in so far 
resemble varieties, often have much restricted ranges. For instance, 
Mr. H. C. Watson has marked for me in the well-sifted London 
Catalogue of plants (4th edition) 63 plailts which are therein ranked 
as species, but which he considers as so closely allied to other 
species as to be of doubtful value : these 63 reputed species range 
on an average over 6*9 of the provinces into which Mr. Watson has 
divided Great Britain. Now, in this same Catalogue, 53 acknow- 
ledged varieties are recorded, and these range over 77 provinces; 


whereas, the species to which these varieties belong range over 14*3 
provinces. So that the acknowledged varieties have nearly the same 
restricted average range, as have the closely allied forms, marked 
for me by Mr. Watson as doubtful species, but which are almost 
universally ranked by British botanists as good and true species. 


Finally, varieties cannot be distinguished from species, — except, 
first, by the discovery of intermediate linking forms ; and, secondly, 
by a certain indefinite amount of difference between them ; for two 
forms, if differing very little, are generally ranked as varieties, not- 
withstanding that they cannot be closely connected; but the amount 
of difference considered necessary to give to any two forms the rank 
of species cannot be defined. In genera having more than the average 
number of species in any cov iitry, the species of these genera have 
more than the average number of varieties. In large genera the 
species are apt to be closely, but unequally, allied together, forming 
little clusters round other species. Species very closely allied to 
other species apparently have restricted ranges. In all these respects 
the species of large genera present a strong analogy with varieties. 
And we can clearly understand these analogies, if species once 
existed as varieties, and thus originated ; whereas, these analogies 
are utterly inexplicable if species are independent creations. 

We have, also, seen that it is the most flourishing or dominant 
species of the larger genera within each class which on an average 
yield the greatest number of varieties ; and varieties, as we shall 
hereafter see, tend to become converted into new and distinct 
species. Thus the larger genera tend to become larger ; and through- 
out nature the forms of life which are now dominant tend to become 
still more dominant by leaving many modified and dominant 
descendants. But by steps hereafter to be explained, the larger 
genera also tend to break up into smaller genera. And thus, the 
forms of life throughout the universe become divided into groups 
subordinate to groupj 



Struggle for Existence. 

Its bearing on natural selection — The term used in a wide sense — Geome- 
trical ratio of increase — Rapid increase of naturalised animals and 
plants — Nature of the checks to increase — Competition universal — • 
Effects of climate — Protection from the number of individuals — 
Complex relations of all animals and plants throughout nature — 
Struggle for life most severe between individuals and varieties of the 
same species: often severe between species of the same genus — The 
relation of organism to organism the most important of all relations. 

Before entering on the subject of this chapter, I must make a few 

preliminary remarks, to show how the struggle for existence bears 

on Natural Selection. It has been seen in the last chapter that 

amongst organic beings in a state of nature there is some individual 

variability : indeed I am not aware that this has ever been disputed. 

It is immaterial for us whether a multitude of doubtful forms be 

called species or sub-species or varieties ; what rank, for instance, the 

two or three hundred doubtful forms of British plants are entitled 

to hold, if the existence of any well-marked varieties be admitted. 

But the mere existence of individual variability and of some fev/ 

well-marked varieties, though necessary as the foundation for the 

work, helps us but little in understanding how species arise in 

nature. How have all those exquisite adaptations of one part of 

the organisation to another part, and to the conditions of life, and 

of one organic being to another being, been perfected? AVe see 

/ these beautiful co-adaptations most plainly in the woodpecker and 

/ the misletoe ; and only a little less plainly in the humblest parasite 

' which clings to the hairs of a q'ladruped or feathers of a bird ; in 

the structure of the beetle which dives through the water : in the 

I plumed seed which is wafted by the gentlest breeze; in short, we 

1 see beautiful adaptations everywhere and in every part of the 

organic world. 

Again, it may be asked, how is it that varieties, which I have 
called incipient species, become ultimately converted into good and 
distinct species, which in most cases obviously differ from each 
other far more than do the varieties of the same species ? How do 
those groups of species, which constitute what aro called distinct 


g;«nera, and which differ from each other more than do the species 
of the same genus, arise ? All these results, as we shall more fully 
see in th© next chapter, follow from the struggle for life. Owing to 
this struggle, variations, however slight, and from whatever cause- 
proceeding, if they be in any degree profitable to the individuals of 
a species, in their infinitely complex relations to other organic 
beings and to their physical conditions of life, will tend to the 
preservation of such individuals, and will generally be inherited 
by the offspring. The offspring, also, wi^l thus have a better 
chance of surviving, for, of the many individuals of any species 
which are periodically born, but a small number can survive. 
I have called this principle, by which each slight variation, if 
useful, is . preserved, by the term Katural Selection, in order to 
mark its relation to man's power of selection. But the expression 
"oTten used by Mr. Herbert Spencer of the Survival of the Fittest 
is more accurate, and is sometimes equally convenient. We have 
seen that man by selection can certainly produce great results, and 
can adapt organic beings to his own uses, through the accumulation 
of slight but useful variations, given to him by the hand of Nature. 
But Natural Selection, as we shall hereafter see, is a power inces- 
santly ready for action, and is as immeasurably superior to man's 
feeble efforts, as the works of Nature are to those of Art. 

We will now discuss in a little more detail the struggle for 
existence. In my future work this subject will be treated, as it 
well deserves, at greater length. The elder De CandoUe and Lyell 
have largely and philosophically shown that all organic beings arc 
exposed to severe competition. In regard to plants, no one has 
treated this subject with more spirit and ability than W. Herbert, 
Dean of Manchester, evidently the result of his great horticultural 
knowledge. Nothing is easier than to admit in words the truth of 
the universal struggle for life, or more diflBcult — at least I have 
found it so — than constantly to bear this conclusion in mind. Yet 
unless it be thoroughly engrained in the mind, the whole economy 
of nature, with every fact on distributiou, rarity, abundance, extinc- 
tion, and variation, will be dimly seen or quite misunderstood. 
We behold the face of nature bright with gladness, we often see 
superabundance of food ; we do not see or we forget, tliat the birds » 
which are idly singing round us mostly live on insects or seeds, and \ 
are thus constantly destroy in g life ; or we forget how largely these 
songsters, or their eggs, or their nestlings, are destroyed by birds 
and beasts of prey ; we do not always bear in mind, that, thougli 
food may be now superabundant, it is not so at all seasons of oBch 
recurring year. 


V r c>L-Cr^ ^'^yK.^-^ 


^ w 



Tlie Term, Struggle for Existence, used in a large sense. 

I should premise that I use this term in a large and metaphorical 
sense inpluding dependence of one being on another, and including 
(which is more important) not only the life of the individual, but 
success in leaving progeny. Two canine animals, in a time of 
dearth, may be truly said to struggle with each other which shall 
get food and live. But a plant on the edge of a desert is said to 
struggle for life against the drought, though more properly it should 
be said to be dependent on the moisture. A plant which annually 
produces a thousand seeds, of which only one on an average comes 
to maturity, may be more truly said to struggle with the plants of 
the same and other kinds which already clothe the ground. The 
•jnisletoe is dependent on the apple and a few other trees, but can 
only in a far-fetched sense be said to struggle with these trees, for, 
if too many of these parasites grow on the same tree, it languishes 
and dies. But several seedling misletoes, growing close together on 
the same branch, may more truly be said to struggle with each 
other. As the misletoe is disseminated by birds, its existence 
depends on them ; and it may metaphorically be said to struggle 
with other fruit-bearing plants, in tempting the birds to devour and 
thus disseminate its seeds. In these several senses, which pass into 
each other, I use for convenience' sake the general term of Struggle 
for Existence. 

Geometrical Ratio of Increase. L p-''''^'^^'t/^ 

A struggle for existence inevitably follows from the high rate at 
which all organic beings tend to increase. Every being, which 
during its natural lifetime produces several eggs or seeds, must suffer 
destruction during some period of its life, and during some season 
or occasional year, otherwise, on the princij'le of geometrical increase, 
its numbers would quickly become so inordinately great that no 
country could support the product. Hence, as more individuals 
are produced than can possibly survive, there must in every case 
be a struggle for existence, either one individual Avitli another of 



same species, 

or with the individuals of distinct species, or 

with the physical conditions of life. It is the doctrine of Malthus 
applied with manifold force to the whole animal and vegetable 
kingdoms ; for in this case there can be no artificial increase of food, 
and no prudential restraint from marriage. Although some species 
may be now increasing, more or less rapidly, in numbers, all cannot 
do so, for the world would not hold them. 
There is no exception to the nile that every organic being 


naturally increases ut so liigli a rate, that, if not destroyed, the 
earth would soon be covered by the progeny of a single pair. Even 
slow-breeding man has doubled in twenty-five years, and at this 
rate, in less than a thousand years, there would literally not be 
standing-room for his progeny. Linna3us has calculated that if an 
annual plant produced only two seeds — and there is no plant so 
unproductive as this — and their seedlings next year produced two, 
and so on, then in twenty years there would be a million plants. 
The elephant is reckoned the slowest breeder of all known animals, 
and I have taken some pains to estimate its probable minimum rate 
of natural increase; it will be safest to assume that it begins 
breeding when thirty years old, and goes on breeding till ninety 
years old, bringing forth six young in the interval, and surviving 
till one hundred years old ; if this be so, after a period of from 
740 to 750 years there would be nearly nineteen million elephants 
alive, descended from tbe first pair. 

But we have better evidence on this subject than mere theoretical 
calculations, namely, the numerous recorded cases of the astonish- 
ingly rapid increase of various animals in a state of nature, when 
circumstances have been favourable to them during Uwo or three 
following seasons. Still more striking is the evidence from oi:r 
domestic animals of many kinds which have run wild in several 
parts of the world ; if the statements of the rate of increase ot 
slow-breeding cattle and horses in South America, and latterly 
in Australia, had not been well authenticated, they would have 
been incredible. So it is with plants; cases could be given of 
introduced plants which have become common throughout whole 
islands in a period of less than ten years. Several of the plants^ 
such as the cardoon and a tall thistle, which are now the com- 
monest over the wide plains of La Plata, clothing square leagues 
of surface almost to the exclusion of every other plant, have been 
introduced from Europe ; and there are plants whicli now range in 
India, as I hear from Dr. Falconer, from Cape Comorin to the 
Himalaya, which have been imported from America since its dis- 
covery. In such cases, and endless others could be given, no one 
supposes, that the fertility of the animals or plants has been suddenly 
and temporarily increased in any sensible degree. The obvious 
explanation is that the conditions of life have been highly favourable, 
and that there has consequently been less destruction of the old and 
voung, and that nearly all the young have been enabled to breedL 
*Their geometrical ratio of increase, the result of which never fails to 
be surprising, simply explains their extraordinarily rapid increase 

d wide diffusion in their new homes. 

E 2 


In a state of nature almost every full-grown plant annually 
produces seed, and amongst animals there are very few which do 
not annually pair. Hence we may confidently assert, that all 
plants and animals are tending to increase at a geometrical ratio, — 
that all would rapidly stock every station in which they could any 
how exist, — and that this geometrical tendency to increase must be 
checked by destruction at some period of life. Our familiarity with 
the larger domestic animals tends, I think, to mislead us : we see 
no great destruction falling on them, but we do not keep in mind 
that thousands are annually slaughtered for food, and that in a state 
of nature an equal number would have somehow to be disposed of. 

The only difference between organisms which annually produce 
eggs or seeds by the thousand, and those which produce extremely 
few, is, that the slow-breeders would require a few more years to 
people, under favourable conditions, a whole district, let it be ever 
so large. The condor lays a couple of eggs and the ostrich a score, 
and yet in the same country the condor may be the more numerous 
of the two ; the Fulmar petrel lays but one egg, yet it is believed 
to be the most numerous bird in the world. One fly deposits 
hundreds of eggs, and another, like the hippobosca, a single one ; 
but this difference does not determine how many individuals of the 
two species can be supported in a district. A large number of eggs 
is of some importance to those species which depend on a fluctua- 
ting amount of food, for it allows them rapidly to increase in 
number. But the real importance of a large number of eggs or 
seeds is to make up for much destruction at some period of life ; 
and this period in the great majority of cases is an early one. If 
an animal can in any way protect its own eggs or young, a small 
number may be produced, and yet the average stock be fully kept 
up ; but if many eggs or young are destroyed, many must be 
produced, or the species will become extinct. It would suffice to 
keep up the full number of a tree, which lived on an average for a 
thousand years, if a single seed were produced once in a thousand 
years, supposing that this seed were never destroyed, and could be 
ensured to germinate in a fitting place. So that, in all cases, the 
average number of any animal or plant depends only indirectly on 
the number of its eggs or seeds. 

In looking at Nature, it is most necessary to keep the foregoing 
considerations always in mind — never to forget that every single 
organic being may be said to be striving to the utmost to increase 
in numbers ; that each lives by a struggle at some period of its life ; 
that neavy destruction inevitably falls either on the young or o^d, 
during each generation or at recurrent intervals. Lighter ani 



check, mitigate the destruction ever so little, and the number of 
the species will almost instantaneously increase to any amount. 

Nature of the Checks to Increase. 

The causes which check the natural tendency of each species to 
.ncrease are most obscure. Look at the most vigorous species ; by 
as much as it swarms in numbers, by so much will it tend to 
increase still further. "We know not exactly what the checks are 
even in a single instance. Nor will this surprise any one who 
reflects how ignorant we are on this head, even in regard to mankind, 
although so incomparably better known than any other animal. This 
subject of the checks to increase has been ably treated by several 
authors, and I hope in a future work to discuss it at considerable 
length, moi'e especially in regard to the feral animals of South 
America. Here I will make only a few remarks, just to recall to 
the reader's mind some of the chief points. Eggs or very young 
animals seem generally to suffer most, but this is not invariably the 
case. With plants there is a vast destruction of seeds, but, from 
some observations which I have made it appears that the seedlings 
jsufifer most from germinating in ground already thickly stocked 
with other plants. Seedlings, also, are destroyed in vast numbers 
by various enemies ; for instance, on a piece of ground three feet 
long and two wide, dug and cleared, and where there could be no 
choking from other plants, I marked all the seedlings of our native 
weeds as they came up, and out of 357 no less than 295, were 
destroyed, chiefly by slugs and insects. If turf which has long been 
mown, and the case would be the same with turf closely browsed 
by quadrupeds, be let to grow, the more vigorous plants gradually 
kill the less vigorous, though fully grown plants ; thus out oi 
twenty species growing on a little plot of mown turf (three feet by 
four) nine species perished, from the other species being allowed to 
grow up freely. 

The amount of food for each species of course gives the extreme 
limit to which each can increase ; but very frequently it is not the 
obtaining food, but the serving as prey to other animals, which 
determines the average numbers of a species. Thus, there seems to 
be little doubt that the stock of partridges, grouse, and hares on 
any large estate depends chiefly on the destruction of vermin. If 
not one head of game were shot during the next twenty years in 
England, and, at the same time, if no vermin were destroyed, there 
would, in all probability, be less game than at present, although 
hundreds of thousands of game animals are now annually shot. On 
the other hand, in some cases, as with the elephant, none are 




destroyed by beasts of prey ; for even the tiger in India mout rarely 
dares to attack a young elephant protected by its dam. 

Climate plays an important part in determining the average 
numbers of a species, and periodical seasons of extreme cold or 
drought seem to be the most effective of all checks. I estimated 
(chiefly from the greatly reduced numbers of nests in the spring) 
that the winter of 1854-5 destroyed four-fifths of the birds in my 
own grounds; and this is a tremendous destruction, when we 
remember that ten per cent, is an extraordinarily severe mortality 
from epidemics with man. The action of climate seems at first 
sight to be quite independent of the struggle for existence ; but m 
s^o far as climate chiefly acts in reducing food, it brings on the most 
severe struggle between the individuals, whether of the same or of 
distinct species, which subsist on the same kind of food. Even 
when climate, for instance extreme cold, acts directly, it will be 
the least vigorous individuals, or those which have got least food 
through the advancing winter, which will suffer most. When we 
travel from south to north, or from a damp region to a dry, we 
mvariably see some species gradually getting rarer and rarer, and 
finally disappearing ; and the change of climate being conspicuous, 
we are tempted to attribute the whole effect to its direct action. 
But this is a false view ; we forget that each species, even where it 
most abounds, is constantly suffering enormous destruction at some 
period of its life, from enemies or from competitors for the same 
place and food ; and if these enemies or competitors be in the least 
degree favoured by any slight change of climate, they will increase 
in numbers ; and as each area is already fully stocked with inhabi- 
tants, the other species must decrease. When we travel south- 
ward and see a species decreasing in numbers, we may feel sure 
that the cause lies quite as much in other species being favoured, as 
in this one being hurt. So it is when we travel northward, but in 
a somewhat lesser degree, for the number of species of all kinds, 
and therefore of competitors, decreases northwards ; hence in going 
northwards, or in ascending a mountain, we far oftener meet with 
Btunted forms, due to the directly injurious action of climate, than we 
do in proceeding southwards or in descending a mountain. When 
we reach the Arctic regions, or snow-capped summits, or absolute 
deserts, the struggle for life is almost exclusively with the elements. 

That climate acts in main part indirectly by favouring other 
species, we clearly see in the prodigious number of plants which 
in our gardens can perfectly well endure our climate, but which 
never become naturalised, for they cannot compete with our native 
plants nor resist destruction by our native animals. 


When a species, owing to highly favourable circumstances, 
increases inordinately in numbers in a small tract, epidemics — at 
least, this seems generally to occur with our game animals — often 
ensue ; and here we have a limiting check independent of the 
struggle for life. But even some of these so-called epidemics 
appear to be due to parasitic worms, which have from some cause, 
possibly in part through facility of diffusion amongst the crowded 
animals, been disproportionally favoured : and here comes in a sort 
of struggle between the parasite and its prey. 

On the other hand, in many cases, a large stock of individuals 
of the same species, relatively to the numbers of its enemies, is 
absolutely necessary for its preservation. Thus we can easily raise 
Ijlenty of corn and rape-seed, &c., in our fields, because the seeds 
are in great excess compared with the number of birds which feed 
on them ; nor can the birds, though having a superabundance of 
food at this one season, increase in number proportionally to the 
bupply of seed, as their numbers are checked during winter; but 
any one who has tried, knows how troublesome it is to get seed 
from a few v^^heat or other such plants in a garden : I have in thiis* 
case lost every single seed. This view of the necessity of a large 
stock of the same species for its preservation, explains, I believe, 
some singular facts in nature such as that of very rare plants being 
sometimes extremely abundant, in the few spots where they do 
exist ; and that of some social plants being social, that is abounding 
in individuals, even on the extreme verge of their range. For in 
such cases, we may believe, that a plant could exist only where the 
conditions of its life were so favourable that many could exist 
together, and thus save the species from utter destruction. I 
should add that the good effects of intercrossing, and the ill effects 
of close interbreeding, no doubt come into play in many of theso 
cases ; but I will not here enlarge on this subject. 

Complex delations of all Animals and Plants to each other tn 
the Struggle for Existence, 

Many cases are on record showing how complex and unexpected 
are the checks and relations between organic beings, which have tc 
struggle together in the same country. I will give only a single 
instance, which, though a simple one, interested me. In Stafford- 
shire, on the estate of a relation, where I had ample means of 
investigation, there was a large and extremely barren heath, which 
had never been touched by the hand of man ; but several hundred 
acres of exactly the same nature had been enclosed twenty-five 
years previously and planted with Scotch fir. The change in the 


Dative vegetation of the planted part of the heath was most 
remarkable, more than is generally seen in passing from one quite 
different soil to another ; not only the proportional numbers of the 
heath-plants were wholly changed, but twelve species of plants (not 
■counting grasses and carices) flourished in the plantations, which 
could not be founsi on the heath. The effect on the insects must 
have been still greater, for six insectivorous birds were very 
nommon in the plantations, which were not to be seen on the 
neath ; and the heath was frequented by two or three distinct 
insectivorous birds. Here we see how potent has been the effect of 
the introduction of a single tree, nothing whatever else having been 
'done, with the exception of the land having been enclosed, so that 
'cattle could not enter. But how important an element enclosure is, 
I plainly saw near Farnham, in Surrey. Here there are extensive 
heaths, with a few clumps of old Scotch firs on the distant hill- 
tops : within the last ten years large spaces have been enclosed, 
and self-sown firs are now springing up in multitudes, so close 
together tliat all cannot live. When I ascertained that these 
young trees had not been sown or planted, I was so much sur- 
prised at their numbers that I went to several points of view, 
v/hence I could examine hundreds of acres of the unenclosed heath, 
and literally I could not see a single Scotch fir, except the old 
planted clumps. But on looking closely between the stems of the 
heath, I found a multitude of seedlings and little trees which had 
been perpetually browsed down by the cattle. In one square yard, 
at a point some hundred yards distant from one of the old clumps, 
I counted thirty-two little trees ; and one of them, with twenty- 
six rings of growth, had, during many years tried to raise its head 
above the stems of the heath, and had failed. No wonder that, as 
soon as the land was enclosed, it became thickly clothed with 
vigorously growing young firs. Yet the heath was so extremely 
barren and so extensive that no one would ever have imagined that 
cattle would have so closely and effectually searched it for food. 

Here we see that cattle absolutely determine the existence of 
the Scotch fir ; but in several parts of the world insects determine the 
existence of cattle. Perhaps Paraguay offers the most curious 
instance of this ; for here neither cattle nor horses nor dogs have 
ever run wild, though they swarm southward and northward in a 
feral state ; and Azara and Rengger have shown that this is caused 
by the greater number in Paraguay of a certain fly, which lays its 
^gs in the navels of these animals when first born. The increase 
of these flies, numerous as they are, must be habitually checked by 
some means, probably by other parasitic insects. Hence, if certain 


insectiTorous birds were to decrease in Faragiia}', the parasitic 
insects would probably increase ; and this would lessen the number 
of the navel-frequenting flies — then cattle and horses would become 
feral, and this would certainly greatly alter (as indeed I have obser- 
ved in parts of South America) the vegetation : this again would 
largely affect the insects; and this, as we have just seen in Stafford- 
shire, the insectivorous birds, and so onwards in ever-increasing 
circles of complexity. Not that under nature the relations will 
ever be as simple as this. Battle within battle must be con- 
tinually recurring with varying success ; and yet in the long- 
run the forces are so nicely balanced, that the face of nature remains 
for long periods of time uniform, though assuredly the merest trifle 
would give the victory to one organic being over another. Never- 
theless, so profound is our ignorance, and so high our presumption, 
that we marvel when we hear of the extinction of an organic being ; 
and as we do not see the cause, we invoke cataclysms to desolate the 
world, or invent laws on the duration of the forms of life ! 

I am tempted to give one more instance showing how plants and 
animals, remote in the scale of nature, are bound together by a web 
of complex relations. I shall hereafter have occasion to show thai 
the exotic Lobelia fulgens is never visited in my garden by insects, 
and consequently, from its peculiar structure, never sets a seed. 
Nearly all our orchidaceous plants absolutely require the visits of 
insects to remove their pollen-masses and thus to fertilise them. I 
find from experiments that humble-bees are almost indispensable to 
the fertilisation of the heartsease (Viola tricolor), for other bees do 
not visit this flower. I have also found that the visits of bees are 
necessary for the fertilisation of some kinds of clover : for instance? 
20 heads of Dutch clover (Trifolium repens) yielded 2,290 seeds, but 
20 other heads protected from bees produced not one. Again, 100 
heads of red clover (T. pl-aTen'S'e) producedr-^TTOCTseeds^ but the same 
number of protected beads produced not a single seed. Humble-bees 
alone visit red clover, as other bees cannot reach the nectar. It has 
been suggested that moths may fertilise the clovers ; but I doubt 
whether they could do so in the case of the red clover, from their 
weight not being sufficient to depress the wing-petals. Hence we 
may infer as highly probable that, if the whole genus of humble-bees 
became extinct or very rare in England, the heartsease and red 
clover would become very rare, or wholly disappear. The number 
of humble-bees in any district depends in a great measure on the 
number of field-mice, which destroy their combs and nests ; and 
CoL Newman, who has long attended to the hab:Us of humble-bees, 


believes that " more than two-thirds of them are thus destroyed all 
over England." Now the number of mice is largely dependent, aa 
every one knows, on the number of cats ; and Col. Newman says, 
" Near villages and small towns I have found the nests of humble- 
bees more numerous than elsewhere, which I attribute to the num- 
ber of cats that destroy the mice." Hence it is quite credible that 
the presence of a feline animal in large numbers in a district might 
determine, through the intervention first of mice and then of bees, 
the frequency of certain flowers in that district ! 

In the case of every species, many different checks, acting at 
different periods of life, and during different seasons or years, pro- 
bably come into play ; some ons check or some few being generally 
the most potent ; but all will concur in determining the average 
number or even the existence of the species. In some cases it can 
be shown that widely-different checks act on. the same species 
in different districts. When we look 2.t the plants and bushes 
clothing an entangled bank, we are tempted to attribute their pro- 
portional numbers and kinds to what we call chance. But how false 
a view is this ! Every one has heard that when an American forest 
is cut down, a very different vegetation springs up ; but it has been 
observed that ancient Indian ruins in the Southern United States, 
which must formerly have been cleared of trees, now display the 
same beautiful diversity and proportion of kinds as in the surround- 
ing virgin forest. What a struggle must have gone on during long 
centuries between the several kinds of trees, each annually scattering 
its seeds by the thousand ; what war between insect and insect — 
between insects, snails, and other animals with birds and beasts of 
prey — all striving to increase, all feeding on each other, or on the 
trees, their seeds and seedlings, or on the other plants which first 
clothed the ground and thus checked the growth of the trees ! Throw 
up a handful of feathers, and all fall to the ground according to 
definite laws ; but how simple is the problem where each shall fall 
compared to that of the action and reaction of the innumerable 
plants and animals which have determined, in the course of cen- 
turies, the proportional numbers and kinds of trees now growing on 
the old Indian ruins ! 

The dependency of one organic being on another, as of a parasite 
on its prey, lies generally between beings remote in the scale of 
nature. This is likewise sometimes the case with those which may 
be strictly said to struggle with each other for existence, as in the 
case of locusts and grass-feeding quadrupeds. But the struggle will 
almost invariably be most severe between the individuals of the 
same species, for they frequent the same districts, require the same 


food, and ojio^vpngpt^ fp {f^ ""^"^Q finrpi"^^'" — In the case ot varieties 
-t5nKe''*same si^ecies, the struggle will generally be almost equally 
severe, and we sometimes see the contest soon decided : for instance, 
if several varieties of wheat 1>3 sown together, and tlie mixed seed be 
resown, some of the varieties which best suit the soil or climate, or are 
naturally the most fertile, will beat the others and so yield more 
seed, and will consequently in a few years supplant the other varie- 
ties. To keep up a mixed stock of even such extremely close varie- 
ties as the variously-coloured sweet-peas, they must be each year 
harvested separately, and the seed then mixed in due proportion, 
otherwise the weaker kinds will steadily decrease in number and 
disappear. So again with the varieties of sheep : it has been asserted 
that certain mountain-varieties will starve out other mountain- 
varieties, so that they cannot be kept together. The same result 
has followed from keeping together different varieties of the medicinal 
leech. It may even be doubted whether the varieties of any of oui 
domestic plants or animals have so exactly the same strength, 
habits, and constitution, that the original proportions cf a mixed 
stock (crossing being prevented) could be kept up for half-a-dozen 
generations, if they were allowed to struggle together, in the same 
manner as beings in a state of nature, and if the seed or young were 
not annually preserved in due proportion. 

Struggle for Life most severe 'between Individuals and Varieties 
of the same Species. 

As the species of the same genus usually have, though by no 
means invariably, much similarity in habits and constitution, and 
always in structure, the struggle will generally be more severe 
betv/een them, if they come into competition with each other, than 
between the species of distinct genera. We see this in the recent 
extension over parts of the United States of one species of swallow 
having caused the decrease of another species. The recent increase 
of the missel-thrush in narts of Scotland has caused the decrease of 
the song-thrush. How frequently we hear of one species of rat 
taking the place of another species under the most different climates ! 
In Kussia the small Asiatic cockroach has everywhere driven before 
It its great congener. In Australia the imported hive-bee is rapidly 
exterminating the small, stingless native bee. One species of char- 
lock has been known to supplant another species ; and so in other 
cases. We can dimly see why the competition should be most severe 
between allied forms, which fill nearly the same place in the economy 
of nature ; but probably in no one case could we precisely say why 
one species has been victorious over another in the great battle of lifo, 


A corollary of the highest importance may be deduced from the 
foregoing remarks, namely, that the structure of every organic being 
is related, in the most essential yet often hidden manner, to that of 
all the other organic beings, with which it comes into competition 
for food or residence, or from which it has to escape, or on which it 
preys. This is obvious in the structure of the teeth and talons of 
the tiger ; and in that of the legs and claws of the parasite which 
clings to the hair on the tiger's body. But in the beautifully plumed 
(seed of the dandelion, and in the flattened and fringed legs of the 
water-beetle, the relation seems at first confined to the elements of 
air and water. Yet the advantage of plumed seeds no doubt stands 
in the closest relation to the land being already thickly clothed with 
other plants ; so that the seeds may be widely distributed and fall 
on unoccupied ground. In the water-beetle, the structure of its 
legs, so well adapted for diving, allows it to compete with other 
aquatic insects, to hunt for its own prey, and to escape serving as 
prey to other animals. 

The store of nutriment laid up within the seeds of many plants 
Beams at first sight to have no sort of relation to other plants. But 
from the strong growth of young plants produced from such seeds* 
as peas and beans, when sown in the midst of long grass, it may bo 
suspected that the chief use of the nutriment in the seed is to favour 
the growth of the seedlings, whilst struggling with other plants 
growing vigorously all around. 

Look at a plant in the midst of its range, why does it not double 
or quadruple its numbers? We know that it can perfectly well 
withstand a little more heat or cold, dampness or dryness, for else- 
where it ranges into slightly hotter or colder, damper or drier dis- 
tricts. In this case we can clearly see that if we wish in imagination 
to give the plant the power of increasing in number, we should have 
to give it some advantage over its competitors, or over the animals 
which prey on it. On the confines of its geographical range, a change 
of constitution with respect to climate would clearly be an advantage 
to our plant ; but we have reason to believe that o^nlv a few plants 
or animals range so far, that they are destroyed exclusively by the 
rigour of the climate. Not until we reach the extreme confines of 
life, in the Arctic regions or on the borders of an utter desert, will 
competition cease. The land may be extremely cold or dry, yet 
there will be competition between some few species, or between the 
individuals of the same species, for the warmest or dampest spots. 

Hence we can see that when a plant or animal is placed in a new 
country amongst new competitors, the conditions of its life will 
generally be changed in an essential manner, although the climate 

ty'UAP. III.] 

f CE. ^ ITS ^ ' 


may be exactlly the same as in its former home. If its average num- 
bers are to iniprease in its new home, we should have to modify it in 
a different way to what we should have had to do in its native 
country ; for we should have to give it some advantage over a different 
set of competitors or enemies. 

It is good thus to try in imagination to give to any one species an 
advantage over another. Probably in no single instance should we 
know what to do. This ought to convince us of oiu: ignorance on 
the mutual relations of all organic beings ; a conviction as necessary, 
as it is difficult to acquire. All that we can do, is to keep steadily 
in mind that each organic being is striving to increase in a geometri- 
cal ratio ; that each at some period of its life, during some season of 
the year, during each generation or at intervals, has to struggle for 
life and to suffer great destruction. When we reflect on this strugglej 
we may console ourstlves with the full belief, that the war of nature 
is not incessant, that no fear is felt, that death is generally prompt,, 
and that the vigorous, the healthy, and the happy survive and 






Natural SrLECTioN; or the Survival op ^/he Fittest. 

Natural Selection — its power compared with man's selection — its power 
on characters of trifling importance — its power at /all ages and on both 
sexes — Sexual Selection — On the generality of /intercrosses between 
individuals of the same species — Circumstances i favourable and unfa- 
vourable to the results of Natural Selection, /lamely, intercrossing, 
isolation, number of individuals — Slow action -A Extinction caused by 
Natural Selection — Divergence of Character, roilated to the diversity of 
inhabitants of any small area, and to naturalisation — Action of Natural 
Selection, through Divergence of Character, and Extinction, on the de- 
scendants from a common parent — Explains the grouping of all organic 
beings — Advance in organisation — Low forms preserved — Convergence 
of character — Indefinite multiplication of species — Summary. 

How will the struggle for existence, briefly discussed in the last 
chapter, act in regard to variation ? Can the principle of selection, 
v/hich we have seen is so potent in the hands of man, apply under 
nature ? I think we shall see that it can act most efficiently. Let 
the endless number of slight variations and individual differences 
occurring in our domestic productions, and, in a lesser degree, in 
those under nature, be borne in mind ; as well as the strength of 
the hereditary tendency. Under domestication, it may be truly 
said that the whole organisation becomes in some degree plastic. 
But the variability, which we almost universally meet with in our 
domestic productions, is not directly produced, as Hooker and 
Asa Gray have well remarked, by man ; he can neither originate 
varieties, nor prevent their occurrence ; he can only preserve and 
accumulate such as do occur. Unintentionally he exposes organic 
beings to new and changing conditions of life, and variability 
ensues ; but similar changes of conditions might and do occur under 
nature. Let it also be borne in mind how infinitely complex and 
close-fitting are the mutual relations of all organic beings to each 
other and to their physical conditions of life; and consequently 
what infinitely varied diversities of structure might be of use to 
each being under changing conditions of life. Can it, then, be 
thought improbable, se^-ing that variations useful to man have 


r undoubtedly occurrod, that other variations usetal in some way to 

I each being in the great and complex battle of life, should occur in 

the course of many successive generations ? If such do occur, can we 

doubt (remembering that many more individuals *are born than can 

. possibly survive) that individuals having any advantage, however 

1 slight, over others, would have the best chance of surviving and of 

i procreating their kind ? On the other hand, we may feel sure that 

any variation in the least degree injurious would bo rigidly de- 

i stroyed. This preservation of favourable individual differences and 

J variations, and the destruction of those which are injurious, I have 
called Natural Selection, or the Survival of the Fittest. Variations 
neither useful nor injurious would not be affected by natural selec- 
tion, and would be left either a fluctuating element, as perhaps we 
see in certain polymorphic species, or would ultimately become 
fixed, owing to the nature of the organism and the nature of the 

Several writers have misapprehended or objected to the term 
Natural Selection. Some have even imagined that natural selection 
induces variability, whereas it implies only the preservation of such 
variations as arise and are beneficial to the being under its con- 
ditions of life. No one objects to agriculturists speaking of the 
potent effects of man's selection ; and in this case the individual 
differences given by nature, which man for some object selects, 
must of necessity first occur. Others have objected that the term 
selection iinplies conscious choice in the animals which become 
modified; and it has even been urged that, as plants have no voli- 
tion, natural selection is not applicable to them! In the literal 
sense of the word, no doubt, natural selection is a false term ; but 
who ever objected to chemists speaking of the elective affinities of 
the various elements ? — and yet an acid cannot strictly be said to 
elect the base with which it in preference combines. It has been 
said that I speak of natural selection as an active power or Deity ; 
but who objects to an author speaking of the attraction of gravity 
as ruling the movements of the planets ? Every one knows what 
is meant and is implied by such metaphorical expressions ; and 
they are almost necessary for brevity. So again it is difficult to 
avoid personifying the word Nature ; b ut I mean by Nature , only 
the aggregjatangtio n and produ ctof many natural laws, and by lawg 
the seq uenci^-a Le vents as ascer tamed byiTs. W ith a little tamiliarify 
iuch superficial objections wilTnbe forgotten. 

We shall best understand the probable course of natural selection 
by taking the case of a country undergoing some slight physical 
change, for instance, of climate. The proportional numbers of its 


inhabitants will almost immediately undergo a change, and some 
species will probably become extinct. We may conclude, from 
what we have seen of the intimate and complex manner in which 
the inhabitants of each country are bound together, that any change 
in the numerical proportions of the inhabitants, independently of 
the change of climate itself, would seriously affect the others. If 
the country were open on its borders, new forms would certainly 
immigrate, and this would likewise seriously disturb the relations 
of some of the former inhabitants. Let it be remembered how 
powerful the influence of a single introduced tree or mammal has 
been shown to be. But in the case of an island, or of a country 
partly surrounded by barriers, into which new and better adapted 
forms could not freely enter, we should then have places in the 
economy of nature which would assuredly be better filled up, if 
some of the original inhabitants were in some manner modified * 
for, had the area been open to immigration, these same places would 
have been seized on by intruders. In such cases, slight modifica- 
tions, which in any way favoured the ii^iividuals of any species, by 
better adapting them to their altered conditions, would tend to be 
preserved ; and natural selection would have free scope for the work 
of improvement. 

We have good reason to believe, as -shown in the first-^^ha^t^, 
that changes in the conditions of life give a tendency to increased 
variability ; and in the foregoing cases the conditions have changed, 
and this would manifestly be favourable to natural selection, by 
affording a better chance of the occurrence of profitable variations. 
Unless such occur, natural selection can do nothing. Under the 
term of " variations," it must never be forgotten that mere indivi- 
dual difi'erences are included. As man can produce a great result 
with his domestic animals and plants by adding up in any given 
direction individual differences, so could natural selection, but far 
more easily, from having incomparably longer tim.e for action. Nor 
do I believe that any great physical change, as of climate, or any un- 
usual degree of isolation to check immigration, is necessary in order 
that new and unoccupied places should be left, for natural selec- 
tion to fill up by improving some of the varying inhabitants. For 
as all the inhabitants of each country are struggling together with 
nicely balanced forces, extremely slight modifications in the struc- 
ture or habits of one species would often give it an advantage over 
others ; and still further modifications of the same kind would often 
still further increase the advantage, as long as the species continued 
Tinder the same conditions of life and profited by similar means of 
subsistence and defence. No country can be named in which all 

Chap. IV.] 



the Dative inhabitants are now so perfectly adapted to each otiiei. 
and to the physical conditions under which they live, that none of 
them could be still better adapted or im]-)roved ; for in all countries, 
the natives have been so far conquered by naturalised productions, 
that they have allowed some foreigners to take firm possession of 
the land. And as foreigners have thus in every country beaten 
feK)me of the natives, we may safely conclude that the natives might 
have been modified with advantage, so as to have better resisted the 

As man can producf^, and certainly has produced, a great result 
by his methodical and unconscious means of selection, what may not 
natural selection effect ? Man can act only on external and visible 
characters : Kature, if I may be allowed to personify the natural 
preservation or survival of the fittest, cares nothing for appearances, 
except in so far as they are useful to any being. She can act on 
every internal organ, on every shade of constitutional difference, 
on the whole machinery of life. Man selects only for his own 
good : Nature only for that of the being which she tends. Every 
selected character is fully exercised by her, as is implied by the fact 
of their selection.'^ Man keejis the natives of many climates in the 
same country ; he seldom exercises each selected character in some 
peculiar and fitting manner ; he feeds a long and a short beaked 
pigeon on the same food; he docs not exercise a long-backed or 
long-legged quadruped in any peculiar manner ; he exposes sheep 
with long and short wool to the same climate. He does not allow 
the most vigorous males to struggle for the females. He does not 
rigidly destroy all inferior animals, but protects during each varying 
season, as far as lies in his power, all his productions. He often 
begins his selection by some half-monstrous form ; or at least by 
some modification prominent enough to catch the eye or to be 
plainly useful to him. Under nature, the slightest differences of 
structure or constitution may well turn the nicely-balanced scale in 
the struggle for life, and so be preserved. How fleeting are th ft ife-tVc "j^ 
wishes and eff or ts of man ! how short his ti me ! and consequently 
h'ow~poor will be his results, compared with those .accumulated by 
Ij atufe dd r ing whole ge ologi^caTperioHi"! Can we wonder, then, that ,' 
Nature's productions shculd be far "Truer" in character than man's 
productions ; that they should be infinitely better adapted to the 
most complex conditions of life, and shoiild plainly bear the stamp 
of far higher workmanship ? 

It may metaphorically be said that natural selection is daily and' 
hourly scrutinising, throughout the world, the slightest variations ; 
rejecting those that are bad, preserving and adding up all that ano 

- — " Y 


good ; silently and insensibly working, wlienever ami loherevtr 
opportunity offers^ at the improvement ofeacli organic Toeing in 
fetalTon to its organic and inorganic conditions of life. We see 
nothing of these slow changes in progress, until the han d of time 
tias- marked the lapse of ages, and then so imperfect is our view into 
long-past geological ages, that we see only that the forms of life are 
ow different from what they formerly were. 

In order that any great amount of modification should be effected 
in a species, a variety when once formed must again, perhaps after 
a long interval of time, vary or present individual differences of the 
same favourable nature as before ; and these must be again pre- 
served, and so onwards step by step. Seeing that individual 
differences of the same kind perpetually recur, this can hardly be 
considered as an unwarrantable assumption. But whether it is 
true, we can judge only by seeing how far the hypothesis accords 
with and explains the general phenomena of nature. On the other 
hand, the ordinary belief that the amount of possible variation is a 
strictly limited quantity is likewise a simple assumption. 

Although natural selection can act only through and for the good 
of each being, yet characters and structures, which we are apt to 
consider as of very trifling importance, may thus be acted on. When 
we see leaf-eating insects green, and bark-feeders mottled-grey; the 
alpine ptarmigan white in winter, the red-grouse the colour of 
heather, we must believe that these tints are of service to these 
birds and insects in preserving them from danger. Grouse, if not 
destroyed at some period of their lives, would increase in countless 
numbers ; they are known to suffer largely from birds of prey ; and 
hawks are guided by eyesight to their prey — so much, so, that on 
parts of the Continent persons are warned not to keep white 
pigeons, as being the most liable to destruction. Hence natural 
selection might be effective in giving the proper colour to each 
kind of grouse, and in keeping that colour, when, once acquired, 
true and constant. Nor ought we to think that the occasional 
destruction of an animal of any particular colour would produce 
little effect : we should remember how essential it is in a flock of 
white sheep to destroy a lamb with the faintest trace of black. 
We have seen how the colour of the hogs, which feed on the 
" paint-root " in Yirginia, determines whether they shall live or die. 
In plants, the down on the fruit and the colour of the flesh are con- 
sidered by botanists as characters of the most trifling importance : 
yet we hear from an excellent horticulturist. Downing, that in the 
United States smooth-skinned fruits suffer far more from a beetle, 
a Curculio, than those with down ; that purple plums suffer fa; 


more Ci-om a certain disease than yellow plums ; whereas another 
disease attacks yellow-fieshcd peaches far more than those with 
other coloured flesh. If, with all the aids of art, these slight differ- 
ences make a great difference in cultivating the several varieties, 
assuredly, in a state of nature, where the trees would have to 
struggle with other trees and with a host of enemies, such differ- 
ences would effectually settle which variety, whether a smooth or 
downy, a yellow or purple fleshed fruit, should succeed. 

In looking at many small points of difference between species, 
which, as far as our ignorance permits us to judge, seem quite 
unimportant, we must not forget that climate, food, &c., have no 
doubt produced some direct effect. It is also necessary to bear in 
mind that, owing to the law of correlation, when one part varies, 
and the variations are accumulated through natural selection, other 
modifications, often of the most unexpected nature, will ensue. 

As we see that those variations which, under domestication appear 
at any particular period of life, tend to reappear in the offspring at 
the same period ; — for instance, in the shape, size, and flavour of 
the seeds of the many varieties of our culinary and agricultural 
plants ; in the caterpillar and cocoon stages of the varieties of the 
silkworm ; in the eggs of poultry, and in the colour of the down of 
their chickens ; in the horns of our sheep and cattle when nearly 
adult ; — so in a state of nature, natural selection will be enabled to 
act on and modify orgaiuc beings at any age, by the accumulation 
of variations profitable at that age, and by their inheritance at a 
corresponding age. If it profit a plant to have its seeds more and 
more widely disseminated by the wind, I can see no greater diffi- 
culty in this being effected through natural selection, than in the 
cotton-planter increasing and improving by selection the down in 
the pods on his cotton-trees. Natural selection may modify and 
adapt the larva of an insect to a score of contingencies, wholly 
different from those which concern the mature insect ; and these 
modifications may affect, through correlation, the structure of the 
adult. So, conversely, modifications in the adult may affect 
the structure of the larva ; but in all cases natural selection will 
ensure that they shall not be injurious: for if they were so, the- 
species would become extinct. 

^~^aLaial selucLiou vvUlrmodify the structure of the young in relation 
to the parent, and of the parent in relation to the young. In social 
animals it will adapt the structure of each individual for the benefit, 
of the whole community ; if the community profits by the selected 
change. What natural selection cannot do, is to modify the struc- 
ture of one species, without giving it any advantage, for the goc<l of 

F i2 


another species ; and though statements to this efifect may be found 
in works of natural history, I cnnnot find one case which will bear 
investigation. A structure used only once in an animal's life, if of 
high importance to it, might be modified to any extent by natural 
selection ; for instance, the great jaws possessed by certain insects, 
used exclusively for opening the cocoon — or the hard tip to the 
beak of unhatched birds, used for breaking the egg. It has been 
asserted, that of the best short-beaked tumbler-pigeons a greater 
number perish in the egg than are able to get out of it ; so that 
fanciers assist in the act of hatching. Now if nature had to make 
the beak of a full-grown pigeon very short for the bird's own advan- 
tage, the process of modification would be very slow, and there 
would be simultaneously the most rigorous selection of all the 
young birds within the egg, which had the most powerful and 
hardest beaks, for all with weak beaks would inevitably perish ; or, 
more delicate and more easily broken shells might be selected, 
the thickness of the shell being known to vary like every other 

It may be well here to remark that with all beings there must be 
much fortuitous destruction, which can have little or no influence 
on the course of natural selection. For instance a vast number of 
eggs or seeds are annually devoured, and these could be modified 
through natural selection only if they varied in some manner which 
protected them from their enemies. Yet many of these eggs or 
seeds would perhaps, if not destroyed, have yielded individuals 
better adapted to their conditions of life than any of those which 
happened to survive. So again a vast number of mature animalfi 
and plants, whether or not they be the best adapted to their con- 
ditions, must be annually destroyed by accidental causes, which 
would not be in the least degree mitigated by certain changes of 
structure or constitution which would in other ways be beneficial to 
the species. But let the destruction of the adults be ever so heavy, 
if the number which can exist in any district be not wholly kept 
down by such causes, — or again let the destruction of eggs or seeds 
be so great that only a hundredth or a thousandth part are developed, 
— yet of those which do survive, the best adapted individuals, sup- 
posing that there is any variability in a favourable direction, will 
tend to propagate their kind in larger numbers than the less well 
adapted. If the numbers be whoUj kept down by the causes just 
indicated, as will often have been the case, natural selection will be 
powerless in certain beneficial directions ; but this is no valid 
objection to its efficiency at other times and in other ways ; for wo 
01*6 far from ha^-ing any reason to suppose that many species ever 


undergo modification and improvement at the same time in the 
same area. 

Sexual Selection. 

Inasmuch as peculiarities often appear under domestication in one 
PCX and become hereditarily attached to that sex, so no doubt it aviU 
be under nature. Thus it is rendered possible for the two sexes to 
be modified through natural selection in relation to different habits 
of life, as is sometimes the case ; or for one sex to be mod' 5ed in 
relation to the other sex, as commonly occurs. This lead? me to 
say a few words on what I have called Sexual Selection. This form of 
selection depends, not on a struggle for existence in relation to other 
organic beings or to external conditions, but on a struggle between 
the individuals of one sex, generally the males, for the possession of 
the other sex. The result is not death to the unsuccessful com- 
petitor, but few or no offspring. Sexual selection is, therefore, less 
rigorous than natural selection. Generally, the most vigorous males, 
those which are best fitted for their places in nature, will leave most 
progeny. But in many cases, victory depends not so much on 
general vigour, as on having special weapons, confined to the male 
sex. A hornless stag or spurless cock would have a poor chance of 
leaving numerous ofispring. Sexual selection, by always allowing 
the victor to breed might surely give indomitable courage, length 
to the spur, and strength to the wing to strike in the spurred leg, in 
nearly the same manner as does the brutal cockfighter by the care- 
ful selection of his best cocks. How low in the scale of nature the 
law of battle descends, I know not ; male alligators have been 
described as fighting, bellou'ing, and whirling round, like Indians in 
a war-dance, for the possession of the females ; male salmons have 
been observed fighting all day long ; male stag-beetles sometimes 
bear wounds from the huge mandibles of other males ; the males 
of certain hymenopterous insects have been frequently seen by that 
inimitable observer M. Fabre, fighting for a particular female who 
sits by, an apparently unconcerned beholder of the struggle, and 
then retires with the conqueror. The war is, perhaps, severest 
between the males of polygamous animals, and these seem oftenest 
provided with special weapons. Tho males of carnivorous animals 
are already well armed; though to them and to others, special 
means of defence may be given through means of sexual selection, 
as the mane to the lion, and the hooked jaw to the male salmon ; 
for the shield may be as important for victory, as the sword or 

Amongst birds, the contest is often of a more peaceful charactex^ 


All those who have attended to the subject, believe that there is the 
severest rivalry between the males of many species to attract, by 
singing, the females. The rock-thrush of Guiana, birds of paradise, and 
some others, congregate ; and successive males display with the most 
elaborate care, and show off in the best manner their gorgeous plu- 
mage; they likewise perform strange antics before the females, which, 
standing by as spectators, at last choose the most attractive partner. 
Those who have closely attended to birds in confinement well know 
that they often take individual preferences and dislikes; thus Sir E, 
Heron has described how a pied peacock was eminently attractive to 
all his hen birds. I cannot Jiere enter on the necessary details ; but 
if man can in a short time give beauty and an elegant carriage to his 
bantams, according to his standard of beauty, I can see no good 
reason to doubt that female birds, by selecting, during thousands of 
generations, the most melodious or beautiful males, according to their 
standard of beauty, might produce a marked effect. Some well- 
known laws, with respect to the plumage of male and fernale birds,^ 
in comparison with the plumage of the young, can partly be ex- 
plained through the action of sexual selection on variations occurring 
at different ages, and transmitted to the males alone or to both sexes 
at corresponding ages ; but I have not space here to enter on this 

Thus it is, as I believe, that when the males and females of any 
animal have the same general habits of life, but differ in structure, 
colour, or ornament, such differences have been mainly caused by 
sexual selection : that is, by individual males having had, in suc- 
cessive generations, some slight advantage over other males, in their 
weapons, means of defence, or charms, which they have transmitted 
to their male offspring alone. Yet, I would not wish to attribute all 
sexual differences to this agency : for we see in our domestic animals 
peculiarities arising and becoming attached to the male sex, which 
apparently have not been augmented through selection by man. 
The tuft of hair on the breast of the wild turkey-cock cannot be of 
any use, and it is doubtful whether it can be ornamental in the eyes 
of the female bird ; — indeed, had the tuft appeared under domestica- 
tion, it would have bsen called a monstrosity. 

Illustrations of the Action of Natural Selection^ or the Survival 

of the Fittest, 

In order to make it clear how, as I believe, natural selection acts, 
I must beg permission to give one or two imaginary illustrations. 
Let us take the case of a wolf, which preys on various animals, 
securing some by craft, some by strength, and some by fleetness j 


and let us suppose that the fleetest prey, a deer for instance, bad 
from any change in the country increased in numbers, or that other 
prey had decreased in numbers, during that season of the year when 
the wolf was hardest pressed for food. Under such circumstances the 
swiftest and slimmest wolves would have the best chance of surviv- 
ing, and so be preserved or selected, — provided always that they 
retained strength to master their prey at this or some other period of 
the year, when they were compelled to prey on other animals. I can 
see no more reason to doubt that this would be the result, than that 
man should be able to improve the fleetuess of his greyhouuds by 
careful and methodical selection, or by that kind of unconscious 
selection which follows from each man trying to keep the best dogs 
without any thought of modifying the breed. I may add, that, 
according to Mr. Pierce, there are two varieties of the wolf inhabiting 
the Catskill Mountains in the Uriited States, one with a light grey- 
hound-like form, which pursues deer, and the other more bulky, 
with shorter legs, which more frequently attacks the shepherd's 

It should be observed that, in the above illustration, I speak of 
the slimmest individual wolves, and not of ariy single strongly- 
marked variation iiaving been preserved. In former editions of ttis 
work I sometimes spoke as if this latter alternative had frequently 
occurred. I saw the great importance of individual differences, and 
this led me fully to discuss the results of unconscious selection by 
man, which depends on the preservation of all the more or less 
valuable individuals, and on the destruction of the worst. I saw, 
also, that the preservation in a state of nature of any occasional 
deviation of structure, such as a monstrosity, would be a rare event ; 
and that, if at first preserved, it would generally be lost by subse- 
quent intercrossing v/ith ordinary individuals. Nevertheless, until 
reading an able and valuable article in the ' North British Eeview ' 
(1867), I did not appreciate how rarely single variations, whether 
slight or strongly-marked, could be perpetuated. The author takes 
the case of a pair of animals, producing during their lifetime two 
hundred offspring, of which, from various causes of destruction, only 
two on an average survive to pro-create their kind. This is rather 
an extreme estimate for most of the higheranimals, butby nomeans 
so for many of the lower organisms. He then shows that if a single 
individual were born, which varied in some manner, giving it twice 
as good a chance of life as that of the other individuals, j'ct the 
chances would be stiongly against its survival. Supposing it to 
tjurvive and to breed, and that half its young inherited the favour- 
able variation ; still, as the Keviewer goes on to show, the young 


would have only a slightly better chance of surviving and bre(3dingi 
and tins chance would go on decreasing in the succeeding genera- 
tions. The justice of these remarks cannot, I think, be disputed. 
if, for instance, a bird of some kind could procure its food more 
easily by having its beak curved, and if one were born with its beak 
strongly curved, and which consequently flourished, nevertheless 
there would be a very poor chance of this one individual perpetuat- 
ing its kind to the exclusion of the common form ; but there can 
hardly be a doubt, judging by what we see taking place under 
domestication, that this result would follow from the preservation 
during many generations of a large number of individuals with more 
or less strongly curved beaks, and from the destruction of a still 
larger number with the straightest beaks. 

^/'Urshould not, however, be overlooked that certain rather strongly 
'marked variations, which no one would rank as mere individual 
differences, frequently recur owing to a similar organisation being 
similarly acted on, — of which fact numerous instances could be 
given with our domestic productions. In such cases, if the varj'ing 
individual did not actually transmit to its offspring its newly-acquired 
character, it would undoubtedly transmit to them, as long as the 
existing conditions remained the same, a still stronger tendency to 
vary in the same manner. There can also be little doubt that the 
tendency to vary in the same manner has often been so strong that 
all the individuals of the same species have been similarly modified 
without the aid of any form of selection. Or only a third, fifth, or 
tenth part of the individuals may have been thus affected, of which 
fact several instances could be given. Thus Graba estimates that 
about one-fifth of the guillemots in the Faroe Islands consist of a 
variety so well marked, tli^t it was formerly ranked as a distinct 
species under the name of Uria lacrymans. In cases of this kind, if 
the variation were of a beneficial nature, the original form would 
soon be supplanted by the modified form, through the survival of 
the fittest. 

To the effects of intercrossing in eliminating variations of all 
kinds, I shall have to recur ; but it may be here remarked that 
most animals and plants keep to their proper homes, and do 
not needlessly wander about ; we see this even with migratory 
birds, which almost always return to the same spot. Consequently 
each newly-formed variety would generally be at first local, as seems 
to be the common rule with varieties in a state of nature ; so that 
similarly modified individuals would soon exist in a small body 
together, and would often breed together. If the new variety were 
successful in its battle for life, it would slowly spread from a centra) 


district, competing with and conquering the unchanged individuals 
on the margins of an ever-increasiag circle. 

It may be worth while to give another and more complex illus- 
tration of the action of natural selection. Certain plants excrete 
sweet juice, apparently for the sake of eliminating something in- 
jurious from the sap : this is effected, for instance, by glands at the 
base of the stipules in some Leguminosa3, and at the backs of the 
leaves of the common laurel. This juice, though small in quantity _, 
is greedily sought by insects ; but their visits do not in any way 
benefit the plant. Now, let us suppose that the juice or nectar was 
excreted from the inside of the flowers of a certain number of plants 
of any species. Insects in seeking the nectar would get dusted with 
pollen, and would often transport it from one flower to another. 
The flowers of two distinct individuals of the same species would 
thus get crossed ; and the act of crossing, as can be fully proved, 
gives rise to vigorous seedlings, which consequently would have the 
best chance of flourishing and surviving. The plants which produced 
flowers with the largest glands or nectaries, excreting most nectar, 
would oftenest be visited by insects, and would oftenest be crossed ; 
and so in the long-run would gain the upper hand and form a local 
variety. The flowers, also, which had their stamens and pistils 
placed, in relation to the size and habits of the particular insect 
which visited them, so as to favour in any degree the transportal of 
the pollen, would likewise be favoured. We might have taken the 
case of insects visiting flowers for the sake of collecting pollen in- 
stead of nectar; and as pollen is formed for the sole purpose of 
fertilisation, its destruction appears to be a simple loss to the plant ; 
yet if a little pollen were carried, at flrst occasionally and then 
habitually, by the pollen-devouring insects from flower to flower, 
and a cross thus effected, although nine-tenths of the pollen were 
destroyed, it might still be a great gain to the plant to be thus 
robbed ; and the individuals which produced more and more pollen, 
and had larger anthers, would be selected. 

When our plant, by the above process long continued, had been 
rendered highly attractive to insects, they would, unintentionally on 
their part, regularly carry pollen from flower to flower ; and that 
they do this effectually, I could easily show by many striking facts. 
I will give only one, as likewise illustrating one step in the separa- 
tion of the sexes of plants. Some hoily-trees bear only male flowers, 
which have four stamens producing a rather small quantity of 
pollen, and a rudimentary pistil ; other holly-trees bear only female 
flowers; these have a full-sized pistil, and four stamens witli 
shrivelled anthers, in which not a grain of pollen can be detected 


IlaTiDg found a female tree exactly sixty yards from a male tree.. 1 
put the stigmas of twenty flowers, taken from different branches, 
under the microscope, and on all, without exception, there were a 
few pollen-grains, and on some a profusion. As the wind had set 
for several days from the femaLe to the male tree, the pollen could 
not thus have been carried. The weather had been cold and 
boisterous, and therefore not favourable to bees, nevertheless every 
female flower which I examined had been effectually fertilised by 
the bees, which had flown from tree to tree in search of nectar. But 
to return to our imaginary case : as soon as the plant had been 
rendered so highly attractive to insects that pollen was regularly 
carried from flower to flower, another process might commence. No 
naturalist doubts the advantage of what has been called the "physio- 
logical division of labour ; " hence we may believe that it would be 
advantageous to a plant to produce stamens alone in one flower or 
on one whole plant, and pistils alone m another flower or on another 
plant. In plants under culture and placed under new conditions of 
life, sometimes the male organs and sometimes the female organs 
become more or less impotent ; now if we suppose this to occur in 
ever so slight a degree under nature, then, as pollen is already 
carried regularly from flower to flower, and as a more complete sepa- 
ration of the sexes of our plant would be advantageous on the prin- 
ciple of the division of labour, individuals with this tendency more 
and more increased, would be continually favoured or selected, 
until at last a complete separation of the sexes might be effected. 
It would take up too much space to show the various steps, 
through dimorphism and other means, by which the separation of 
the sexes in plants of various kinds is apparently now in progress ; 
but I may add that some of the species of holly in North America, 
are, according to Asa Gray, in an exactly intermediate condition, or, 
as he expresses it, are more or less dioeciously polygamous. 

Let us now turn to the nectar- feeding insects ; we may suppose 
the plant, of which we have been slowly increasing the nectar by 
continued selection, to be a common plant ; and that certaiu 
insects depended in main part on its nectar for food. I could give 
many facts showing how anxious bees are to save time : for 
instance, their habit of cutting holes and sucking the nectar at 
the bases of certain flowers, which with a very little more trouble, 
they can enter by the mouth. Bearing such facts in mind, it may 
be believed that under certain circumstances individual differences 
in the curvature or length of the proboscis, &c., too slight to be 
appreciated, by us, might profit a bee or other insect, so that 
certain individuals would be able to obtain their food more qmckly 


than others ; and thus the communities to which they belonged 
would flourish and throw off many swarms inheriting the same 
peculiarities. The tubes of the corolla of the common red and 
incarnate cloveis (Trifolium pratcnse and incarnatum) do not on a 
hasty glance appear to differ in length ; yet the hive-bee can easily 
Buck the nectar out of the incarnate clover, but not out of the 
common red clover, which is visited by humble-bees alone ; so that 
whole fields of the red clover offer in vain an abundant supply of 
precious nectar to the hive- bee. That this nectar is much liked by 
the hive-bee is certain ; for I have repeatedly seen, but only in the 
autumn, many hive-bees sucking the flowers through holes bitten 
in the base of the tube by humble-bees. The difference in the 
length of the corolla in the two kinds of clover, which determines 
the visits of the hive-bee, must be very trifling ; for I have been 
assured that when red clover has been mown, the flowers of the 
second crop are somewhat smaller, and that these are visited by 
many hive-bees. I do not know whether this statement is accu- 
rate; nor whether another published statement can be trusted, 
namely, that the Ligurian bee, which is generally considered a 
mere variety of the common hive-bee, and which freely crosses with 
it, is able to reach and suck the nectar of the red clover. Thus, in 
a country where this kind of clover abounded, it might be a great 
advantage to the hive-bee to have a slightly longer or differently 
constructed proboscis. On the other hand, as the fertility of this 
clover absolutely depends on bees visiting the flowers, if humble- 
bees were to become rare in any country, it might be a great 
advantage to the plant to have a shorter or more deeply divided 
corolla, so that the hive-bees should be enabled to suck its flowers. 
Thus I can understand how a flower and a bee might slowly 
become, either simultaneously or one after the other, modified and 
adapted to each other in the most perfect manner, by the con- 
tinued preservation of all the individuals which presented slight 
deviations of structure mutually favourable to each other. 

I am well aware that this doctrine of natural selection, exempli- 
fied in the above imaginary instances, is open to the same objections 
which were first urged against Sir Charles Lyell's noble views on 
" the modern changes of the earth, as illustrative of geology ; " but 
we now seldom hear the agencies which we see still at work, spoken 
of as trifling or insignificant, when used in explaining the excavation 
of the deepest valleys or the formation of long lines of inland 
cliffs. Natural selection acts only by the preservation and accumu- 
lation of small inherited modifications, each profitable to the pre- 
served beinqi and as modern geology has almost banished such 


views as the excavation of a great valley by a single diluvial wave, 
so will natural selection banish the belief of the continued creation 
of new organic beings, or of any great and sudden modification in 
their structure. 

On the Intercrossing of Individuals. 

I must here introduce a short digression. In the case of animals 
and plants with separated sexes, it is of com'se obvious that two 
individuals must always (with the exception of the curious and 
not well-understood cases of parthenogenesis) unite for each birth ; 
but in the case of hermaphrodites this is far from obvious. 
Nevertheless there is reason to believe that with all hermaphrodites 
two individuals, either occasionally or habitually, concur for the 
reproduction of their kind. This view was long ago doubtfully 
suggested by Sprengel, Knight and Kolreuter. We shall presently 
see its importance ; but I must here treat the subject with extreme 
brevity, though I have the materials prepared for an ample dis- 
cussion. All vertebrate animals, all insects, and some other large 
groups of animals, pair for each birth. Modern research has much 
diminished the number of supposed hermaphrodites, and of real 
hermaphrodites a large number pair ; that is, two individuals 
regularly unite for reproduction, which is all that concerns us. 
But still there are many hermaphrodite animals which certainly do 
not habitually pair, and a vast majority of plants are hermaphro- 
dites. What reason, it may be asked, is there for supposing in 
these cases that tv/o individuals ever concur in reproduction ? As 
it is impossible here to enter on details, I must trust to some 
general considerations alone. 

In the first place, I have collected so large a body of facts, and 
made so many experiments, showing, in accordance with the almost 
universal belief of breeders, that with animals and plants a cross 
between different varieties, or between individuals of the same 
variety but of another strain, gives vigour and fertility to the off- 
spring ; and on the other hand, that close interbreeding diminishes 
vigour and fertility ; that these facts alone incline me to believe 
that it is a general law of nature that no organic being fertilises 
itself for a perpetuity of generations; but that a cross with another 
individual is occasionally — perhaps at long intervals of time — 

On the belief that this is a law of nature, we can, I think, under- 
etand several large classes of facts, such as the following, which 
on any other view are inexplicable. Every hybridizer knows how 
unfavourable exposure to wet is to the fertilisation of a flower, yel 


what a multitude of flowers have their anthers and stigmas fully 
exposed to the weather ! If an occasional cross be indispensable, 
notwithstanding that the plant's own anthers and pistil stand so 
near each other as almost to ensure self-fertilisation, the fullest 
freedom for the entrance of pollen from another individual will 
explain the above state of exposure of the organs. Many flowers, 
on the other hand, have their organs of fructification closely 
enclosed, as in the great papilionaceous or pea-family ; but these 
almost invariably present beautiful and curious adaptations in 
relation to the visits of insects. So necessary are the visits of bees 
to many papilionaceous flowers, that their fertility is greatly dimi- 
nished if these visits be prevented. Now, it is scarcely possible for 
insects to fly from flower to flower, and not to carry pollen from 
one to the other, to the great good of the plant. Insects act like a 
camel-hair pencil, and it is suflicient, to ensure fertilisation, just to 
touch with the same brush the anthers of one flower and then the 
stigma of another ; but it must not be supposed that bees would 
thus produce a multitude of hybrids between distinct species ; for il 
a plant's own pollen and that from another species are placed on 
the same stigma, the former is so prepotent that it invariably and 
completely destroys, as has been shown by Gartner, the influence 
of the foreign pollen. 

When the stamens of a flower suddenly spring towards the pistil, 
or slowly move one after the other towards it, the contrivance 
seems adapted solely to ensure self-fertilisation ; and no doubt it i-s 
useful for this end : but the agency of insects is often required to 
cause the stamens to spring forward, as Kolreuter has shown to be 
the case with the barberry ; and in this very genus, which seems to 
have a special contrivance for self-fertilisation, it is well known 
that, if closely-allied forms or varieties are planted near each other, 
it is hardly possible to raise pure seedlings, so largely do they 
naturally cross. In numerous other cases, far from self-fertilisation 
being favoured, there are special contrivances which effectually 
prevent the stigma receiving pollen from its own flower, as I could 
show from the works of Sprengel and others, as well as from my 
own observations : for instance, in Lobelia fulgens, there is a really 
beautiful and elaborate contrivance by which all the infinitely 
numerous pollen-granules are swept out of the conjoined anthers of 
each flower, before the stigma of that individual flower is ready to 
receive them ; and as this flower is never visited, at least in my 
garden, by insects, it never sets a seed, though by placing pollen 
from one flower on the stigma of another, I raised plenty of seed- 
lings. Another species of Lobelia, w^hich is visited by bees, seeds 


freely in my garden. In very many other cases, though there is no 
special meciianical contrivance to prevent the stigma receiving 
pollen from the same flower, yet, as Sprcngel, and more recently 
Hildebrand, and others, have shown, and as I can confirm, either 
the anthers burst befce the stigma is ready for fertihsation, or the 
stigma is ready before the pollen of that flower is ready, so that 
these so-named dichogamous plants have in fact separated sexes, 
and must habitually be crossed. So it is with the reciprocally 
dimorphic and trimorphic plants previously alluded to. How 
strange are these facts ! How strange that the pollen and stigmatic 
surface of the same flower, though placed so close together, as if for 
the very purpose of self-fertilisation, should be in so many cases 
mutually useless to each other ? How simply are these facts ex- 
plained on the view of an occasional cross with a distinct individual 
being advantageous or indispensable ! 

If several varieties of the cabbage, radish, onion, and of some 
other plants, be allowed to seed near each other, a large majority of 
the seedlings thus raised turn out, as I have found, mongrels : for 
instance, I raised 233 seedling cabbages from some plants of different 
varieties growing near each other, and of these only 78 were true to 
their kind, and some even of these were not perfectly true. Yet 
the pistil of each cabbage-flower is surrounded not only by its own 
six stamens, but by those of the many other flowers on the same 
plant ; and the pollen of each flower readily gets on its own stigma 
without insect-agency ; for I have found that plants carefully 
protected from insects produce the full number of pods. How, 
then, comes it that such a vast number of the seedlings are mon- 
grelized ? It must arise from the pollen of a distinct variety having 
a prepotent effect over the flower's own pollen ; and that this is 
part of the general law of good being derived from the intercrossing 
of distinct individuals of the same species. When distinct species 
are crossed the case is reversed, for a plant's own pollen is almost 
always prepotent over foreign pollen ; but to this subject we shall 
return in a future chapter. 

In the case of a large tree covered with innumerable flowers, it 
may be objected that pollen could seldom be carried from tree to 
tree, and at most only from flower to flower on the same tree ; and 
flowers on the same tree can be considered as distinct individuals 
only in a limited sense. I believe this objection to be valid, but 
that nature has largely provided against it by giving to trees a 
strong tendency to bear flowers with separated sexes. When the 
sexes are separated, althougJl the male and female flowers may be 
produced on the same tree, pollen must be regularly carried from 


flower to flower ; and this will give a better chance of pollen being 
occasionally carried from tree to tree. That trees belonging to all 
Orders have their sexes more often separated than other plants, I 
find to be the case in this country ; and at my request Dr. Hooker 
tabulated the trees of New Zealand, and Dr. Asa Gray those of the 
United States, and the result was as I anticipated. On the other 
hand, Dr. Hooker informs me that the rule does not hold good in 
Australia ; but if most of the Australian trees are dichcgamous, the 
same result would follow as if they bore flowers with separated 
sexes. I have made these few remarks on trees simply to call 
attention to the subject. 

Turning for a brief space to animals : various terrestrial species 
are hermaphrodites, such as the land-moUusca and earth-worms ; 
but these all pair. As yet I have not found a single terrestrial 
animal which can fertilise itself. This remarkable fact, which offers 
so strong a contrast with terrestrial plants, is intelligible on the view 
of an occasional cross being indispensable ; for owing to the nature of 
the fertilising element there are no means, analogous to the action 
of insects and of the wind with plants, by which an occasional cross 
could be effected with terrestrial animals without the concurrence of 
two individuals. Of aquatic animals, there are many self-fertilising 
hermaphrodites ; but here the currents of water offer an obvious 
means for an occasional cross. As in the case of flowers, I have as 
yet failed, after consultation with one of the highest authorities, 
namely. Professor Huxley, to discover a single hermaphrodite animal 
with the organs of reproduction so perfectly enclosed that access 
from without, and the occasional influence of a distinct individual, 
can be shown to be physically impossible. Cirripedes long appeared 
to me to present, under this point of view, a case of great difiSculty ; 
but I have been enabled, by a fortunate chance, to prove that two 
individuals, though both are self-fertilising hermaphrodites, do 
sometimes cross. 

It must have struck most naturalists as a strange anomaly that, 
Ixjth with animals and plants, some species of the same family and 
even of the same genus, though agreeing closely with each other in 
their whole organisation, are hermaphrodites, and some unisexual. 
Bnt if, in fact, all hermaphrodites do occasionally intercross, the 
difi'erence between them and unisexual species is, as far as function 
is concerned, very small. 

From these several considerations and from the many special 
facts which I have collected, but which I am unable here to give, 
it appears that with animals and plants an occasional intercross 
between distinct individuals is a very general, if not universal, law 
of nat,iLre. 


Circumstances favourable for the production of new forms 
through Natural Selection. 

This is an extremely intricate subject. A great amount of varia- 
bility, under which term individual differences are always includedj 
will evidently be favourable. A large number of individuals, by 
giving a better chance within any given period for the appearance 
of profitable variations, will compensate for a lesser amount of 
variability in each individual, and is, I believe, a highly important 
element of success. Though Nature grants long periods of time for 
the work of natural selection, she does not grant an indefinite 
period ; for as all organic beings are striving to seize on each place 
in the economy of nature, if any one species does not become modi- 
fied and improved in a corresponding degree with its competitors, it 
will be exterminated. Unless favourable variations be inherited by 
some at least of the offspring, nothing can be effected by natural 
selection. The tendency to reversion may often check or prevent the 
work ; but as this tendency has not prevented man from forming 
by selection numerous domestic races, why should it prevail against 
natural selection ? 

In the case of methodical selection, a breeder selects for some 
definite object, and if the individuals be allowed freely to intercross, 
his work will completely fail. But when many me\i, without 
intending to alter the breed, have a nearly common standard of 
perfection, and all try to procure and breed from the best animals, 
improvement surely but slowly follows from this unconscious 
process of selection, notwithstanding that there is no separation of 
selected individuals. Thus it will be under nature ; for within a 
confined area, with some place in the natural polity not perfectly 
occupied, all the individuals varying in the right direction, though 
in different degrees, will tend to be preserved. But if the area be 
large, its several districts will almost certainly present different 
conditions of life ; and then, if the same species undergoes modifi- 
cation in different districts, the newly-formed varieties will intercross 
on tbiB confines of each. But we shall see in the sixth chapter that 
intermediate varieties, inhabiting intermediate districts, will in the 
long run generally be supplanted by one of the adjoining varieties. 
A. Intercrossing will chiefly affect those animals which unite for each 

' birth and wander much, and which do not breed at a very quick 

rate. Hence with animals of this nature, for instance, birds, 
varieties will generally be confined to separated countries ; and this 
I find to be the case. With hermaphrodite organisms which cross 
only occasionally, and likewise with animals which unite for each 




birtb, but which wander little and can increase at a rapid rate, a 
new and improved variety might be quickly formed on any one 
spot, and might there maintain itself in a body and afterwards 
spread, so that the individuals of the new variety would chiefly 
cross together. On this principle, nurserymen always prefer saving 
seed from a large body of plants, as the chance of intercrossing is 
thus lessened. 

Even with animals which unite for each birth, and which do not 
propagate rapidly, we must not assume that free intercrossing would 
always eliminate the effects of natural selection ; for I can bring 
forward a considerable body of facts showing that within tlie same 
area, two varieties of the same animal may long remain distinct, 
from haunting different stations, from breeding at slightly different 
seasons, or from the individuals of each variety preferring to pair 

Intercrossing plays a very important part in nature by keeping 
the individuals of the same species, or of the same variety, true and 
imiform in character. It will obviously thus act far more efficiently 
with those animals which unite for each birth ; but, as already stated, 
we have reason to believe that occasional intercrosses take place with 
all animals and plants. Even if these take place only at long inter- 
vals of time, the young thus produced will gain so much in vigour 
and fertility over the offspring from long-continued self- fertilisation, 
that they will have a better chance of surviving and propagating 
their kind ; and thus, in the long run, the influence of crosses, even 
at rare intervals, will be great. With respect to organic beings 
extremely low in the scale, which do not propagate sexually, nor 
conjugate, and which cannot possibly intercross, uniformity of cha- 
racter can be retained by them under the same conditions of life, 
only through the principle of inheritance, and through natural selec- 
tion which will destroy any individuals departing from the proper 
type. If ihe conditions of life change and the form undergoes modifi- 
cation, uniformity of character can be given to the modified offspring, 
solely by natural selection preserving similar favourable variations. 

Isolation, also, is an important element in the modification of 
species through natural selection. In a confined or isolated area, if 
not very large, the organic and inorganic conditions of life will 
generally be almost uniform ; so that natural selection will tend to 
modify all the varying individuals of the same species in the same 
manner. Intercrossing with the inhabitants of the surrounding dis- 
tricts will, also, be thus prevented. Moritz Wagner has lately pub- 
lished an interesting essay on this subject, and has shown that the 
service rendered by isolation in preventing crosses between newly- 



foiined varieties is probably greater even than I supposed. But 
from reasons already assigned I can by no means agree with this 
naturalist, that migration and isolation are necessary elements for 
the formation of new species. The importance of isolation is like- 
wise great in preventing, after any physical change in the conditions, 
such as of climate, elevation of the land, &c., the immigration of 
better adapted organisms; and thus new places in the natural 
economy of the district will be left open to be filled up by the 
modification of the old inhabitants. Lastly, isolation will give time 
for a new variety to be improved at a slow rate ; and this may some- 
times be of much importance. If, however, an isolated area be very 
small, either from being surrounded by barriers, or from having very 
peculiar physical conditions, the total number of the inhabitants 
will be small ; and this will retard the production of new species 
through natural selection, by decreasing the chances of favourable 
variations arising. 

The mere lapse of time by itself does nothing, either for or against 
natural selection. I state this because it has been erroneously 
asserted that the element of time has been assumed by me to play 
an all-important part in modifying species, as if all the forms of life 
were necessarily undergoing change through some innate law. Lapse 
of time is only so far important, and its importance in this respect 
is great, that it gives a better chance of beneficial variations arising, 
and of their being selected, accumulated, and fixed. It likewise 
lends to increase the direct action of the physical conditions of life, 
in relation to the constitution of each organism. 

If we turn to nature to test the truth of these remarks, and look 
at any small isolated area, such as an oceanic island, although the 
number of species inhabiting it is small, as we shall see in our 
chapter on Geographical Distribution ; yet of these species a very 
large proportion are endemic, — that is, have been produced there, 
and nowhere else in the world. Hence an oceanic island at first 
sight seems to have been highly favourable for the production of 
new species. But we may thus deceive ourselves, for to ascertain 
whether a small isolated area, or a large open area like a continent, 
has been most favourable for the production of new organic forms, 
we ought to make the comparison within equal times ; and this we 
are incapable of doing. 

Although isolation is of great importance in the production of new 
species, on the whole I am inclined to believe that largeness of area 
is still more important, especially for the production of species which 
shall prove capable of enduring for a long period, and cf spreading 
widely. Throughout a great and open area, not only will there be a 


better chance of favourable variations, arising from the large number 
of individuals of the same species there supported, but the conditions 
of life are much more complex from the large number of already- 
existing species ; and if some of these many species become modified 
and improved, others will have to be improved in a corresponding 
degree, or they will be exterminated. Each new form, also, as soon 
as it has been much improved, will be able to spread over the open 
and continuous area, and will thus come into competition with 
many other forms. Moreover, great areas, though now continuous, 
will often, owing to former oscillations of level, have existed in r, 
broken condition ; so that the good effects of isolation will generally, 
to a certain extent, have concurred. Finally, I conclude that, 
although small isolated areas have been in some respects highly 
favourable for the production of new species, yet that the course of 
modification will generally have been more rapid on large areas ; 
and what is more important, that the new forms produced on large 
areas, which already have been victorious over many competitors, 
will be those that will spread most widely, and will give rise to the 
greatest number of new varieties and species. They will thus play 
a more important part in the .changing history of the organic 

, In accordance with this view, we can, perhaps, understand some 
facts which will be again alluded to in our chapter on Geographical 
Distribution ; for instance, the fact of the productions of the smaller 
continent of Australia now yielding before those of the larger 
Europa30-Asiatic area. Thus, also, it is that continental productions 
have everywhere become so largely naturalised on islands. On a 
small island, the race for life will have been less severe, and there 
will have been less modification and less extermination. Hence, we 
can understand how it is that the flora of Madeira, according to 
Oswald Heer, resembles to a certain extent the extinct tertiary flora 
of Europe. All fresh-water basins, taken together, make a small 
area compared with that of the sea or of the land. Consequently, 
the competition between fresh- water productions will have been less 
severe than elsewhere ; new forms will have been then more slowly 
produced, and old forms more slowly exterminated. And it is in 
fresh-water basins that we find seven genera of Ganoid fishes, 
remnants of a once preponderant order : and in fresh water we find 
some of the most anomalous forms now known in the world, as tho 
Ornithorhynchus and Lepidosiren, which, like fossils, connect to a 
certain extent orders at present widely sundered in the natural 
scale. These anomaliDUS forms may be called living fossils; they 
have endured to the present day, from having inhabited a confined 

o 2 


area, and from having been exposed to less varied, and therefore less 
severe, competition. 

To sum up, as far as the extreme intricacy of the subject permits, 
the circumstances favourable and anfavourable for the production of 
new species through natural selection. I conclude that for terrestrial 
productions a large continental area, which has undergone many 
oscillations of level, will have been the most favourable for the pro- 
duction of many new forms of life, fitted to endure for a long time 
and to spread widely. Whilst the area existed as a continent, the 
inhabitants will have been numerous in individuals and kinds, and 
will have been subjected to severe competition. "When converted 
by subsidence into large separate islands, there will still have existed 
many individuals of the same species on each island : intercrossing 
on the confines of the range of each new species will have been 
checked : after physical changes of any kind, immigration will have 
been prevented, so that new places in the polity of each island will 
have had to be filled up by the modification of the old inhabitants ; 
and time will have been allowed for the varieties in each to become 
well modified and perfected. When, by renewed elevation, the 
islands were reconverted into a continental area, there will again 
have been very severe compciition : the most favoured or improved 
varieties will have been enabled to spread : there will have been 
much extinction of the less improved forms, and the relative propor- 
tional numbers of the various inhabitants of the reunited continent 
will again have been changed ; and again there will have been a fair 
field for natural selection to improve still further the inhabitants, 
and thus to produce new species. 

That natural selection generally acts with extreme slowness I fully 
admit. It can act only when there are places in the natural polity 
of a district which can be better occupied by the modification of 
some of its existing inhabitants. The occurrence of such places will 
often depend on physical changes, which generally take place very 
slowly, and on the immigration of better adapted forms being pre- 
vented. As some few of the old inhabitants become modified, 
tne mutual relations of others will often be disturbed ; and this 
will create new places, ready to be filled up by better adapted forms; 
but all this will take place very slowly. Although all the indi- 
viduals of the same species differ in some slight degree from each 
other, it would often be long before differences of the right nature 
in various parts of the organisation might occur. The result would 
often be greatly retarded by free intercrossing. Many will exclaim 
that these several causes are amply sufficient to neutralise the power 
of natural selection. I do not believe so. But I do believe that 


natural selection will generally act very slowly, only at long intervals 
of time, and only on a few of the inhabitants of the same region. I 
further believe that these slow, intermittent results accord well 
with what geology tells us of the rate and manner at which the in- 
habitants of the world have changed. 

Slow though the process of selection may be, if feeble man can 
do much by artificial selection, I can see no limit to the amount of 
change, to the beauty and complexity of the coadaptations between 
all organic beings, one with another and with their physical con- 
ditions of life, which may have been effected in the long course of 
time through nature's power of selection, that is by the survival 
of the fittest. 

Extinction caused hy Natural Selection. 

This subject will be more fully discussed in our chapter on 
Geology ; but it must here be alluded to from being intimately con- 
nected with natural selection. Natural selection acts solely through 
the preservation of variations in some way advantageous, which 
consequently endure. Owing to the high geometrical rate of increase 
of all organic beings, each area is already fully stocked with inhabit- 
ants ; and it follows from this, that as the favoured forms increase in 
number, so, generally, will the less favoured decrease and become 
rare. Earity, as geology tells us, is the precursor to extinction. We 
can see that any form which is represented by few individuals will 
run a good chance of utter extinction, during great fluctuations in 
the nature of the seasons, or from a temporary increase in the number 
of its enemies. But we may go further than this ; for, as new forms 
are produced, unless we admit that specific forms can go on indefi- 
nitely increasing in number, many old forms must become extinct. 
That the number of specific forms has not indefinitely increased, 
geology plainly tells us ; and we shall presently attempt to show 
why it is that the number of species throughout the world has not 
"become immeasurably great. 

We have seen that the species which are most numerous in indi- 
viduals have the best chance of producing favourable variations 
within any given period. We have evidence of this, in the facts 
stated in the second chapter, showing that it is the common and 
diffused or dominant species which offer the greatest number of 
recorded varieties. Hence, rare species will be less quickly modified 
or improved within any given i)eriod ; they will consequently bo 
beaten in the race for life by the modified and improved descendants 
of the commoner species. 

From these several considerations I think it inevitably follows, 


that as new species in the course of time are formed through natural 
selection, others will become rarer and rarer, and finally extinct. 
The forms which stand in closest competition with those undergoing 
modification and Improvement, will naturally suffer most. And we 
have seen in the chapter on the Struggle for Existence that it is the 
most closely-allied forms, — varieties of the same species, and species 
of the same genus or of related genera, — which, from having nearly 
the same structure, constitution, and habits, generally come into 
the severest competition with each other ; consequently, each new 
variety or species, during the progress of its formation, will generally 
press hardest on its nearest kindred, and tend to exterminate them. 
We see the same process of extermination amongst our domesticated 
productions, through the selection of improved forms by man. 
Many curious instances could be given showing how quickly new 
breeds of cattle, sheep, and other animals, and varietieij of flowers, 
take the place of older and inferior kinds. In Yorkshire, it is 
historically known that the ancient black cattle were displaced by 
the long-horns, and that these "were swept away by the short- 
horns *' (I quote the words of an agricultural writer) " as if by s'>^ae 
murderous pestilence." 

Divergence of Character. 

The principle, which I have designated by this term, is of high 
importance, and explains, as I believe, several important facts. In 
the first place, varieties, even strongly-marked ones, though having 
somewhat of the character of species — as is shown by the hopeless 
doubts in many cases how to rank them — yet certainly differ far 
less from each other than do good and distinct species. Keverthe- 
less, according to my view, varieties are species in the process of 
formation, or are, as I have called them, incipient species. How, 
then, does the lesser difference between varieties become augmented 
into the greater difference between species ? That this does habitu- 
ally happen, we must infer from most of the innumerable species 
throughout nature presenting well-marked differences ; whereas 
varieties, the supposed prototypes and parents of future well-marked 
species, present slight and ill-defined differences. Mere chance, as 
we may call it, might cause one variety to differ in some character 
from its parents, and the offspring of this variety again to differ 
from its parent in the very same character and in a greater degree ; 
but this alone would never account for so habitual and large 
a degree of difference as that between the species of the same 

As has always been my practice, I have sought light on this 


head from our domestic productions. We shall here find something 
analogous. It will be admitted that the production of races so 
different as short-horn and Hereford cattle, race and cart horses, 
the several breeds of pigeons, &c., could never have been effected by 
the mere chance accumulation of similar variations during many 
successive generations. In practice, a fancier is, for instance, struck 
by a pigeon having a slightly shorter beak; another fancier is 
struck by a pigeon having a rather longer beak; and on the 
acknowledged principle that " fanciers do not and will not admire 
a medium standard, but like extremes," they both go on (as has 
actually occurred with the sub-breeds of the tumbler-pigeon) 
choosing and breeding from birds with longer and longer beaks, or 
with shorter and shorter beaks. Again, we may suppose that at an 
early period of history, the men of one nation or district required 
swifter horses, whilst those of another required stronger and bulkier 
horses. The early differences would be very slight ; but, in the 
course of time, from the continued selection of swifter horses in 
the one case, and of stronger ones in the other, the differences would 
become greater, and would be noted as forming two sub-breeds. 
Ultimately, after the lapse of centuries, these sub-breeds would 
become converted into two well-established and distinct breeds. As 
the differences became greater, the inferior animals v;ith interme- 
diate characters, being neither very swift nor very strong, would 
not have been used for breeding, and will thus have tended to dis- 
appear. Here, then, we see in man's productions the action of what 
may be called the principle of divergence, causing differences, at 
first barely appreciable, steadily to increase, and the breeds to 
diverge in character, both from each other and from their common 

But how, it may be asked, can any analogous principle apply in 
nature ? I believe it can and does apply most efficiently (though it 
was a long time before I saw how), from the simple circumstance 
that the more diversified the descendants from any one species 
become in structure, constitution, and habits, by so much will they 
bo better enabled to seize on many and widely diversified places 
in the polity of nature, and so be enabled to increase in numbers. 

We can clearly discern this in the case of animals with simple 
habits. Take the case of a carnivorous quadruped, of which the 
number that can be supported in any country has long ago arrived 
at its full average. If its natural power of increase be allowed to 
act, it can succeed in increasing (the country not undergoing any 
change in conditions) only by its varying descendants seizing on 
places at present occupied by ether animals: some of them, foi 


instance, being enabled to feed on new kinds of prey, either dead 
or alive ; some inhabiting now stations, climbing trees, frequenting 
water, and some perhaps becoming less carnivorous. The more 
diversified in habits and structure the descendants of our carnivo- 
rous animals become, the more places they will be enabled to occupy. 
What applies to one animal will apply throughout all time to all 
animals — that is, if they vary— for otherwise natural selection car.. 
eiiect nothing. So it will be with plants. It has been experi- 
mentally proved, that if a plot of ground be sown with one species 
oi grass, and a similar plot be sown with several distinct genera of 
grasses, a greater number of plants and a greater weight of dry 
herbage can be raised in the latter than in the former case. The 
same has been found to hold good when one variety and several 
mixed varieties of wheat have been sown on equal spaces of ground. 
Hence, if any one species of grass were to go on varyimi;, and the 
varieties were continually selected which differed from each other 
in the same manner, though in a very slight degree, as do the 
distinct species and genera of grasses, a greater number of individual 
plants of this species, including its modified descendants, would 
succeed in living on the same piece of ground. And we know that 
each species and each variety of grass is annually sowing almost 
countless seeds ; and is thus striving, as it may be said, to the 
utmost to increase in number. Consequently, in the course of many 
thousand generations, the most distinct varieties of any one species 
of grass would have the best chance of succeeding and of increasing 
in numbers, and thus of supplanting the less distinct varieties ; and 
varieties, when rendered very distinct from each other, take the rank 
of species. 

The truth of the principle that the greatest amount of life can be 
supported by great diversification of structure, is seen under many 
natural circumstances. In an extremely small area, especially if 
freely open to immigration, and where the contest between indivi- 
dual and individual must be very severe, we always find great 
diversity in its inhabitants. For instance, I found that a piece of 
turf, three feet by four in size, which had been exposed for many 
years to exactly the same conditions, supported twenty species of 
plants, and these belonged to eighteen genera and to eight orders, 
which shows how much these plants differed from each other. So 
it is with the planis and insects on small and uniform islets : also 
in small ponds of fresh water. Farmers find that they can raise 
most food by a rotation of plants belonging to the most different 
orders : nature; follows what may be called a simultaneous rotation. 
Most of the animals and plants which live close round any small 


piece of ground, could live on it (supposing its nature not to be in 
any way peculiar), and may be said to be striving to the utmost to 
live there ; but, it is seen, that where they come into the closest 
competition, the advantages of diversification of structure, with the 
accompanying dififerences of habit and constitution, determine 
that the inhabitants, which thus jostle each other most closely, 
shall, as a general rule, belong to what we call different genera 
and orders. 

The same principle is seen in the naturalisation of plants through 
man's agency in foreign lands. It might have been expected that 
the plants which would succeed in becoming naturalised in any 
land would generally have been closely allied to the indigenes ; for 
these are commonly looked at as specially created and adapted 
for their own country. IL might also, perhaps, have been expected 
rhat naturalised plants would have belonged to a few groups more 
especially adapted to certain stations in their new homes. But the 
case is very different ; and Alph. de Candolle has well remarked, in 
his great and admirable work, that floras gain by naturalisation, 
proportionally with the number of the native genera and species, 
far more in new genera than in new species. To give a single 
instance: in the last edition of Dr. Asa Gray's * Manual of the 
Flora of the Northern United States,' 2G0 naturalised plants are 
enumerated, and these belong to 162 genera. We thus see that 
these naturalised plants are of a highly diversified nature. They 
differ, moreover, to a large extent, from the indigenes, for out of the 
102 naturalised genera, no less than 100 genera are not there indi- 
genous, and thus a large proportional addition is made to the genera 
now living in the United States. 

By considering the nature of the plants or animals which have in 
any country struggled successfully with the indigenes, and have 
there become naturalised, we may gain some crude idea in what 
manner some of the natives would have to be modified, in order to 
gain an advantage over their compatriots; and we may at least 
infer that diversification of structure, amounting to new generic 
differences, would be profitable to them. 

The advantage of diversification of structure in the inhabitants 
of the same region is, in fact, the same as that of the physiological 
division of labour in the organs of the same individual body — a 
subject so well elucidated by Milne Edwards. No physiologist 
doubts that a stomach adapted to digest vegetable matter alone, or 
flesh alone, draws most nutriment from these substances. So in the 
general economy of any land, the more widely and perfectly the 
animals and plants are diversified for different habits of life, so will 


a greater number of individuals be capable of there supporting 
themselves. A set of animals, with their organisation but little 
diversified, could hardly compete with a set more perfectly diversified 
in structure. It may be doubted, for instance, whether the Austra- 
lian marsupials, which are divided into groups diff'eriDg but little 
from each other, and feebly representing, as Mr. Waterhouse and 
others have remarked, our carnivorous, ruminant, and rodent mam- 
mals, could successfully compete with these well-developed orders. 
In the Australian mammals, we see the process of diversification in 
an early and incomplete stage of development. 

The ProhaUe Effects of the Action of Natural Selection through 
Divergence of Character and Extinction^ on the Descendants of 
a Common Ancestor. 

After the foregoing discussion, which has been much compressed, 
wo may assume that the modified descendants of any one species 
will succeed so much* the better as they become more diversified in 
structure, and are thus enabled to encroach on places occupied by 
other beings. Now let us see how this principle of benefit being 
derived from divergence of character, combined with the principles 
of natural selection and of extinction, tends to act. 

The accompanying diagram will aid us in understanding this 
rather perplexing subject. Let A to L represent the species of a 
genus large in its own country ; these species are supposed to 
resemble each other in unequal degrees, as is so generally the case 
in nature, and as is represented in the diagram by the letters 
standing at unequal distances. I have said a large genus, because 
as we sav/ in the second chapter, on an average more species vary in 
large genera than in small genera ; and the varying species of the 
large genera present a greater number of varieties. We have, also, 
seen that the species, which are the commonest and the most widely 
diffused, vary more than do the rare and restricted species. Let (A) 
be a common, widely-diffused, and varying species, belonging to a 
genus large in its own country. The branching and diverging 
dotted lines of unequal lengths proceeding from (A), may represent 
its varying offspring. The variations arc supposed to be extremely 
slight, but of the most diversified nature ; they are not supposed all 
to appear simultaneously, but often after long intervals of time ; nor 
are they all supposed to endure for equal periods. Only those 
variations which are in some way profitable will be preserved or 
naturally selected. And here the importance of the principle of 
benefit derived from divergence of character comes in ; for this will 
generally lead to the most different cr divergent variations (repre- 


sented by the outer dotted lines) being preserved and accumulated 
by natural selection. When a dotted line reaches one of the hori- 
zontal lines, and is there marked by a small numbered letter, a 
sufficient amount of variation is sup[iused to have been accumulated 
to form it into a fairly well-mark ed ^ variety , such as would be 
thought worthy of recor d in a system atic work. 

The intervals between the horizontal lines in the diagram, may 
represent each a tho usand or more generations. After a thousand 
generations, species (A) is supposecTTo' Have produced two fairly 
well-marked varieties, namely a} and rn}. These two varieties will 
generally still be exposed to the same conditions which made their 
parents variable, and the tendency to variability, is injtself Jieredi- 
ta ry ; consequently they will likewise tend to vary, and commonly 
in nearly the same manner as did their parents. ^Moreover, these 
two varieties, being only slightly modified forms, will tend to inherit 
those advantages which made their parent (A) more numerous than 
most of the other inhabitants of the same country ; they will also 
partake of those more general advantages which made the genus to 
which the jmrent-species belonged, a large genus in its own country. 
And all these circumstances are favourable to the production of new 

If, then, these two varieties be variable, the most divergent of 
their variations will generally be yireserved during the next thousand 
generations. And after this interval, variety a^ is supposed in the 
diagram to have produced variety a^, which will, owing to the prin- 
ciple of divcrg.en,cc, differ more from CA) than did variety a}. 
Yariefym* is supposed to have produced two varieties, namely m^ 
and s^, differing from each other, and more considerably from their 
common parent (A). We may continue the process by similar 
steps for any length of time ; some of the varieties, after each 
thousand generations, producing only a single variety, but in a more 
and more modified condition, some producing two or three varieties, 
and some failing to produce any. Thus the varieties or modified 
descendants of the common parent (A), will generally go on 
increasing in number and diverging in character. In the diagram 
the process is represented up to the ten-thousandth generation, and 
under a condensed and simplified form up to the fourteen-thousandth 

But I must here remark that I do not suppose that the process 
ever goes on so regularly as is represented in the diagram, though in 
itself made somewhat irregular, nor that it goes on continuously ; it 
is far more probable that each form remjiins for long periods unal- 
tered, and then again undergoes modification.^ Nor dc I suppose 


that the most divergent varietiss are invariably preserved: a 
medium form may often long endure, and may or may not produce 
more than one modified descendant; for natural selection will 
always act according to the nature of the places which are either 
unoccupied or not perfectly occupied by other beings ; and this will 
depend on infinitely complex relations. But as a general rule, the 
more diversified in structure the descendants from any one species 
can be rendered, the more places they will be enabled to seize on, 
and the more their modified progeny will increase. In our diagram 
the line of succession is broken at regular intervals by small num- 
bered letters marking the successive forms which have become 
sufficiently distinct to be recorded as varieties. But these breaks 
are imaginary, and might have been inserted anywhere, after inter- 
vals long enough to allow the accumulation of a considerable amount 
of divergent variation. 

As all the modified descendants from a common and widely- 
diffused species, belonging to a large genus, will tend to imrtake 
of the same advantages which made their parent successful in 
life, they will generally go on multiplying in number as well as 
diverging in character : this is represented in the diagram by the 
several divergent branches proceeding from (A). The modified 
offspring from the later and more highly improved branches in the 
lines of descent, will, it is probable, often take the place of, and so 
destroy, the earlier and less improved branches : this is represented 
in the diagram by some of the lower branches not reaching to the 
upper horizontal lines. In some cases no doubt the process of 
modification v/ili be confined to a single line of descent, and the 
number of modified descendants will not be increased ; although 
the amount of divergent modification rnay have been augmented. 
This case would he represented in the diagram, if all the lines pro- 
ceeding from (A) were removed, excepting that from a} to a^^. In 
the same way the English race-horse and English pointer have appa- 
rently both gone on slowly diverging in character from their original 
stocks, without either having given off any fresh branches or races. 

After ten thousand generations, species (A) is supposed to have 
produced three forms, a^°,/^°, and m^°, which, from having diverged 
in character during the successive generations, will have come to 
differ largely, but perhaps unequally, from each other and from 
their common parent. If we suppose the amount of change be- 
tween each horizontal line in our diagram to be excessively small, 
these three forms may still be only well-marked varieties ; but we 
have only to suppose the steps in the process of modification to be 
more numerous or greater in amount, to convert these three forms 


Id to doubtful or at last into well-defined species. Thus the diagram 
illustrates the steps by which the small differences distinguishing 
varieties are increased into the larger differences distinguishing spe- 
cies. By continuing the same process for a greater number of gene- 
rations (as shown in the diagram in a condensed and simplified 
manner), we get eight species, marked by the letters between a^* 
and m^*j all descended from (A). Thus, as I believe, species are 
multiplied and genera are formed. 

In a large genus it is probable that more than one species would 
vary. In the diagram I have assumed that a second species (I) has 
produced, by analogous steps, after ten thousand generations, either 
two well-marked varieties (w^^ and 2^°) or two species, according to 
the amount of change supposed to be represented between the hori- 
zontal lines. After fourteen thousand generations, six new species, 
marked by the letters n^^ to z^'^, are supposed to have been produced. 
In any genus, the species which are already very difi'erent in cha- 
racter from each other, will generally tend to produce the greatest 
number of modified descendants ; for these will have the best 
chance of seizing on new and widely different places in the polity 
of nature : hence in the diagram I have chosen the extreme species 
(A), and the nearly extreme species (I), as those which have largely 
varied, and have given rise to new varieties and species. The other 
nine species (marked by capital letters) of our original genus, may 
for long but unequal periods continue to transmit unaltered de- 
scendants ; and this is shown in the diagram by the dotted lines 
unequally prolonged upwards. 

But during the process of modification, represented in the dia- 
gram, another of our principles, namely that of extinction, will have 
played an important part. As in each fully stocked country natural 
selection necessarily acts by the selected form having some advan- 
tage in the struggle for life over other forms, there will be a constant 
tendency in the improved descendants of any one species to sup- 
glantand exterminat^e in each stage of descent their predecessors 
and their original progenitor. For it should be remembered that 
the competition will generally be most severe between those forms 
which are most nearly related to each other in habits, constitution, 
and structure. Hence all the intermediate forms between the earlier 
and later states, that is between the less and more improved states 
of the same species, as well as the original parent-species itself, will 
generally tend to become extinct. So it probably will be with many 
whole collateral lines of descent, which will be conquered by later 
and improved lines. If, however, the modified ofi'spring of a species 
get into some distinct country, or become quickly adapted to some 

State HisTf^ric;;-?! a^-.O 


puite new station, in which offspring and progenitor do not come 
-nto competition, both may continue to exist. 

If, then, our diagram be assumed to represent a considerable 
amount of modification, species (A) and all the earlier varieties will 
have become extinct, being replaced by eight new species (a}^ to 
m") ; and species (I) will be replaced by six {71^* to 2") new species. 

But we may go further than this. The original species of our 
genus were supposed to resemble each other in unequal degrees, as 
is so generally the case in nature ; species (A) being more nearly 
related to B, 0, and D, than to the other species ; and species (1) 
more to G, H, K, L, than to the others. These two species (A) and 
(T) were also supposed to be very common and widely diffused 
species, so that they must originally have had some advantage over 
most of the other species of the genus. Their modified descendants, 
fourteen in number at the fourteen-thousandth generation, will 
probably have inherited some of the same advantages : they have 
also been modified and improved in a diversified manner at each 
stage of descent, so as to have become adapted to many related 
places in the natural economy of their country. It seems, therefore, 
extremely probable that they will have taken the places of, and 
thus exterminated, not only their parents (A) and (I), but likewise 
some of the original species which were most nearly related to their 
parents. Hence very few of the original species will have trans- 
mitted offspring to the fourteen-thousandth generation. We may 
suppose that only one (F), of the two species (E and F) which were 
least closely related to the other nine original species, has trans- 
mitted descendants to this late stage of descent. 

The new species in our diagram descended from the original 
eleven species, will now be fifteen in number. Owing to the diver- 
gent tendency of natural selection, the extreme amount of difference 
in character between species a^* and 2'* will be much greater than 
that between the most distinct of the original eleven species. The 
new species, moreover, will be allied to each other in a widely dif- 
ferent manner. Of the eight descendants from (A) the three marked 
a}\ 2^^, p^\ will be nearly related from having recently branched off 
from a^^ ; h^'^, and /^*, from having diverged at an earlier period from 
o^, will be in some degree distinct from the three first-named species ; 
and lastly, 0^^, e^^, and m^*, will be nearly related one to the other, 
but, from having diverged at the first commencement of the process 
of modification, will be widely different from the other five species, 
and may constitute a sub-genus or a distinct genus. 

The six descendants from (I) will form two sub-genera or genera. 
But as the original spicies (I) differed largely from (A), standing 


nearly at the extreme end of the original genus, the six descendants 
from (I) will, owing to inheritance alone, differ considerably from 
the eight descendants from (A) ; the two groups, moreover, are 
supposed to have gone on diverging in different directions. The 
intermediate species, also (and this is a very important considera- 
tion), which connected the original species (A) and (I), have all 
become, excepting (F), extinct, and have left no descendants. 
Hence the six new species descended from (I), and the eight de- 
scended from (A), will have to be ranked as very distinct genera, 
or even as distinct sub-families. 

Thus it is, as I believe, that two or more genera are produced 
by descent with modification, from two or more species of the same 
genus. And the two or more parent-species are supposed to be 
descended from some one species of an earlier genus. In our dia- 
gram, this is indicated by the broken lines, beneath the capital 
letters, converging in sub-branches downwards towards a single 
point ; this point represents a species, the supposed progenitor of our 
several new sub-genera and genern.. 

It is worth while to reflect for a moment on the character of the 
new species r^^, which is supposed not to have diverged much in 
character, but to have retained the form of (F), either unaltered or 
altered only in a slight degree. In this case, its affinities to the 
other fourteen new species will be of a curious and circuitous nature. 
Being descended from a form which stood between the parent-species 
(A) and (I), now supposed to be extinct and unknown, it will be 
in some degree intermediate in character between the two groups 
descended from these two species. But as these two groups have 
gone on diverging in character from the type of their parents, the 
new species (f^-*) will not be directly intermediate between them, 
but rather between types of the two groups ; and every naturalist 
will be able to call such cases before his mind. 

In the diagram, each horizontal line has hitherto been supposed 
to represent a thousand generations, but each may represent a 
million or more generations ; it may also represent a section of the 
successive strata of the earth's crust including extinct remains. We 
shall, when we come to our chapter on Geology, have to refer again 
to this subject, and I think we shall then see that the diagram 
throws light on the affinities of extinct beings, which, though gene- 
rally belonging to the same orders, families, or genera, with those 
now living, yet are often, in some degree, intermediate in character 
oetween existing groups ; and we can understand this fact, for the 
extinct species lived at various remote epochs when the branching 
lines of descent had diversred less. 


I see no reason to limit the process of modification, as now ex- 
plained, to the formation of genera alone. If, in the diagram, we 
suppose the amount of change represented by each successive group 
of diverging dotted lines to be great, the forms marked a}^ to p^^, 
those marked b^'^ and f^\ and those marked o" to m^^, will form 
three very distinct genera. We shall also have two very distincfc 
genera descended from (I), differing widely from the descendants 
of (A). These two groups of genera will thus form two distinct 
families, or orders, according to the amount of divergent modifica- 
tion supposed to be represented in the diagram. And the two new 
families, or crders, are descended from two species of the original 
genus, and these are supposed to be descended from some still more 
ancient and unknown form. 

We have seen that in each country it is the species belonging 
to the larger genera which oftenest present varieties or incipient 
species. This, indeed, might have been expected ; for, as natural 
selection acts through one form having some advantage over other 
forms in the struggle for existence, it will chiefly act on those which 
already have some advantage; and the largeness of any group 
shows that its species have inherited from a common ancestor some 
advantage in common. Hence, the struggle for the production of 
new and modified descendants will mainly lie between the larger 
groups which are all trying to increase in number. One large group 
will slowly conquer another large group, reduce its numbers, and 
thus lessen its chance of further variation and improvement. 
Within the same large group, the later and more highly perfected 
sub-groups, from branching out and seizing on many new places in 
the polity of Nature, will constantly tend to supplant and destroy 
the earlier and less improved sub-groups. Small and broken groups 
and sub-groups will finally disappear. Looking to the future, we 
can predict that the groups of organic beings which are now large 
and triumphant, and which are least broken up, that is, which have 
as yet suffered least extinction, will, for a long period, continue to 
increase. But which groups will ultimately prevail, no man can 
predict ; for we know that many groups, formerly most extensively 
developed, have now become extinct. Looking still more remotely 
to the future, we mny predict that, owing to the continued and 
steady increase of the larger groups, a multitude of smaller groups 
will become utterly extinct, and leave no modified descendants; 
and consequently that, of the species living at any one period, 
extremely few will transmit descendants to a remote futurity. 1 
shall have to return to this subject in the chapter on Classification, 
but I may add that as, according to this view, extremely few of the 


more ajicient species have transmitted descendants to the present 
day, and, as all the descendants of the same species form a class, we 
can miderstand how it is that there exists so few classes in each 
main division of the animal and vegetable kingdoms. Although 
few of the most ancient species have left modified descendants, yet, 
at remote geological periods, the earth may have been almost as 
well peopled with species of many genera, families, orders, and 
classes, as at the present time. 

On the Degree to which Organisation tends to advance. 

Natural Selection acts exclusively by the preservation and accu- 
mulation of variations, which are beneficial under the organic and 
inorganic conditions to which each creature is exposed at all periods 
of life. The ultimate result is that each creature tends to become 
more and more improved in relation to its conditions. This im- 
provement inevitably leads to the gradual advancement of the 
organisation of the greater number of living beings throughout the 
world. But here we enter on a very intricate subject, for naturalists 
have not defined to each other's satisfaction what is meant by an 
advance in organisation. Amongst the vertebrata the degree of 
intellect and an approach in structure to man clearly come into 
play. It might be thought that the amount of change which the 
various parts and organs pass through in their development from 
the embryo to maturity would suffice as a standard of comparison ; 
but there are cases, as with certain parasitic crustaceans, in which 
several parts of the structure become less perfect, so that the mature 
animal cannot be called higher than its larva. Von Baer's standard 
seems the most widely applicable and the best, namely, the amount 
of differentiation of the parts of the same organic being, in the 
adult state as I should be inclined to add, and their specialisa- 
tion for different functions ; or, as Milne Edwards would express it, 
the compkteness__of Jihe dLvision of physiological labour. . But we 
shall see how obscure this subject is if we look, for instance, to fishes, 
amongst which some naturalists rank those as highest which, like 
the sharks, approach nearest to amphibians ; whilst other naturalists 
rank the common bony or teleostean fishes as the highest, inasmuch 
as they are most strictly fish-like, and differ most from the other 
vertebrate classes. We see still more plainly the obscurity of the 
subject by turning to plants, amongst which the standard of intel- 
lect is of course quite excluded ; and here some botanists rank those 
plants as highest which have every organ, as sepals, petals, stamens, 
and pistils, fully developed in each flower ; whereas other botanists, 



probably with more truth, look at the plants which have theii 
HBveral organs much modified and reduced in number as the 

If we take as the standard of high organisation the amount ol 
differentiation and specialisation of the several organs in each being 
when adult (and this will include the advancement of the brain for 
intellectual purposes), natural selection clearly leads towards this 
standard : for all physiologists admit that the specialisation of organs, 
inasmuch as in this state they perform their functions better, is an 
advantage to each being ; and hence the accumulation of variations 
tending towards specialisation is within the scope of natural selec- 
tion. On the other hand, we can see, bearing in mind that all 
organic beings are striving to increase at a high ratio and to seize on 
every unoccupied or less well occupied place in the economy of 
nature, that it is quite possible for natural selection gradually to fit 
a being to a situation in which several organs would be superfluous 
or useless : in such cases there would be retrogression in the scale of 
organisation. Whether organisation on the whole has actually 
advanced from the remotest geological periods to the present day 
will be more conveniently discussed in our chapter on Geologic al 

But it may be objected that if all organic beings thus tend to 
rise in the scale, how is it that throughout the world a multitude of 
the lowest forms still exist ; and how is it that in each great class 
some forms are far more highly developed than others? iWhy have 
not the more highly developed forms everywhere supplanted and 
exterminated the lower ? Lamarck, who believed in an innate and 
inevitable tendency towards perfection in all organic beings, seems 
to have felt this difficulty so strongly, that he was led to suppose 
that new and simple forms are continually being produced by spon- 
taneous generation. Science has not as yet proved the truth of 
this belief, whatever the future may reveal. On our theory the 
continued existence of lowly organisms offers no difficulty; for 
natural selection, or the survival of the fittest, does not necessarily 
include progressive development — it only takes advantage of such 
variations as arise and are beneficial to each creature under its com- 
plex relations of life. | And it may be asked what advantage, as far 
as we can see, would it be to an infusorian animalcule — to an in- 
testinal worm — or even to an earth-worm, to be highly organised. 
If it were no advantage, these forms would be left, by natural selec- 
tion, unimproved or but little improved, and might remain for 
indefinite ages in their present lowly condition. And geology tells 
us that some of the lowest forms, as the infusoria and rhizopods. 


have remained lor an enormous period in nearly their present state. 
But to suppose that most of the many now existing low forms Lave 
not in the least advanced since the first dawn of life would bo 
extremely rash ; for every naturalist who has dissected seme of the 
beings now ranked as very low in the scale, must have been struck 
with their really wondrous and beautiful organisation. 

Nearly the same remarks are applicable if we look to the different 
grades of organisation within the same great group ; for instance, 
in the vertebrata, to the co-existence of mammals and fish — amongst 
mammalia, to the co-existence of man and the ornithorhynchus — 
amongst fishes, to the co-existence of the shark and the lancelet 
(Amphioxus), which latter fish in the extreme simplicity of its 
structure approaches the invertebrate classes. But mammals and 
fish hardly come into competition with each other ; the advance- 
ment of the whole class of mammals, or of certain members in this 
class, to the highest grade would not lead to their taking the place 
of fishes. Physiologists believe that the brain must be bathed by 
warm blood to be highly active, and this requires aerial respiration ; 
so that warm-blooded mammals when inhabiting the water lie 
under a disadvantage in having to come continually to the sur- 
face to breathe. With fishes, members of the shark family would 
not tend to supplant the lancelet ; for the lancelet, as I hear 
from Fritz Miiller, has as sole companion and competitor on the 
barren sandy shore of South Brazil, an anomalous annelid. The 
three lowest orders of mammals, namely, marsupials, edentata, and 
rodents, co-exist in South America in the same region with nume- 
rous monkeys, and probably interfere little with each other. 
.Although organisation, on the whole, may have advanced and be 
still advancing throughout the world, yet the scale will always 
present many degrees of perfection ; for the high advancement of 
certain whole classes, or of certain members of each class, does not 
at all necessarily lead to the extinction of those groups with which 
they do not enter into close competition. In some cases, as we 
'shall hereafter see, lowly organised forms appear to have been pre- 
served to the present day, from inhabiting confined or peculiar 
stations, where they have been subjected to less severe competitioii, 
and where their scanty numbers have retarded the chance of favour- 
able variations arising. 

Finally, I believe that many lowly organised forms now exist 
throughout the world, from various causes. In some cases varia- 
tions or individual differences of a favourable natui'e may ne^^j: 
have arisen for natural selection to act on and accumulate. In re 
case, probably, has time sufficed for the utmost possible arrjoiurt oi. 

H 2 3 >ii' 


developmeiit. In some few cases there has been what we must call 
retrogression of organisation. But the main cause lies in the fact 
that under very simple conditions of life a high organisation would 
be of no service, — possibly would be of actual disservice, as being 
of a mere delicate nature, and more liable to be put out of order 
and injured. 

Looking to the first dawn of life, when all organic beings, as we 
may believe, presented the simplest structure, how, it has been 
asked, could the first steps in the advancement or differentiation of 
parts have arisen? Mr. Herbert Spencer would probably answer 
that, as soon as simple unicellular organism came by growth or 
division to be compounded of several cells, or became attached to 
any supporting surface, his law " that homologous units of any 
order become differentiated in proportion as their relations to inci- 
dent forces become different " would come into action. But as we 
have no facts to guide us, speculation on the subject is almost useless. 
It is, however, an error to suppose that there would be no struggle 
for existence, and, consequently, no natural selection, until many 
forms had been produced : variations in a single species inhabiting 
an isolated station might be beneficial, and thus the whole mass of 
individuals might be modified, or two distinct forms might arise. 
But, as I remarked towards the close of the Introduction, no one 
ought to feel surprise at much remaining as yet unexplained on the 
origin of species, if we make due allowance for our profound igno- 
rance on the mutual relations of the inhabitants of the world at the 
present time, and still more so during past ages. 

Convergence of Character. 

Mr. H. C. Watson thinks that I have overrated the importance 
of divergence of character (in which, however, he apparently 
believes), and that convergence, as it may be called, has likewise 
played a part. If two species, belonging to two distinct though 
allied genera, had both produced a large number of new and diver- 
gent forms, it is conceivable that these might approach each other 
so closely that they would have all to be classed under the same 
genus ; and thus the descendants of two distinct genera would con- 
verge into one. But it would in most cases be extremely rash to attri- 
bute to convergence a close and general similarity of structure in the 
modified descendants of widely distinct forms. The shape of a crystal 
is determined solely by the molecular forces, and it is not surprising 
tlki'oidissimilar substances should sometimes assume the same form; 
but with organic beings we should bear in mind that the form of 

* C < < c 


eacli depends on an infinitude of complex relations, namely on tha 
variations which have arisen, these being due to causes far too 
intricate to be followed out, — on the nature of the variations which 
have been preserved or selected, and this depends on the surround- 
ing physical conditions, and in a still higher degree on the sur- 
rounding organisms with which each being has come into competi- 
tion, — and lastly, on inheritance (in itself a fluctuatiog element) from 
innumerable progenitors, all of which have had their forms Aeter- 
mined through equally complex relations. It is incredible that the 
descendants of two organisms, which had originally differed in a 
marked manner, should ever afterwards converge so closely as to 
lead to a near approach to identity throughout their whole organi- 
sation. If this had occurred, we should meet with the same form, 
independently of genetic connection, recurring in widely separated 
geological formations ; and the balance of evidence is opposed to 
any such an admission. 

Mr. Watson has also objected that the continued action of natural 
selection, together with divergence of character, would tend to make 
an indefinite number of specific forms. As far as mere inorganic con- 
ditions are concerned, it seems probable that a sufficient number of 
species would soon become adapted to all considerable diversities 
of heat, moisture, &c. ; but I fully admit that the mutual relations 
of organic beings are more important ; and as the number o^ species 
in any country goes on increasing, the organic conditions of life 
must become more and more complex. Consequently there seems 
at first sight no limit to the amount of profitable diversification of 
structure, and therefore no limit to the number of species which 
might be produced. We do not know that even the most prolific 
area is fully stocked with specific forms : at the Cape of Good Hope 
and in Australia, which support such an astonishing number of 
species, many European plants have become naturalised. But 
geology shows us, that from an early part of the tertiary period the 
number of species of shells, and that from the middle part of this 
same period the number of mammals, has not greatly or at all 
increased. What then checks an indefinite increase in the number 
of species? The amount of life (I do not mean the number of 
specific forms) supported on an area must have a limit, depending 
so largely as it does on physical conditions ; therefore, if an area be 
inhabited by very man}'- species, each or nearly each I'pecies will be 
represented by few individuals ; and such species will be liable to 
extermination from accidental fluctuations in the nature of the 
seasons or in the number of their enemies. The process of extermi- 
nation in such cases would be rapid, whereas the production of new 


species must always be slow. Imagine the extreme case of as many 
species as individuals in England, and the first severe winter or 
very dry summer would exterminate thousands on thousands of 
species. Rare species, and each species will become rare if the 
number of species in any country becomes indefinitely increased, 
will, on the principle often explained, present within a given period 
few favourable variations ; consequently, the process of giving birth 
to new specific forms would ,thus be retarded. When any species 
becomes very rare, close interbreeding will help to exterminate it ; 
authors have thought that this comes into play in accounting for 
the deterioration of the Aurochs in Lithuania, of Eed Deer in Scot- 
land, and of Bears in Norway, &c. Lastly, and this I am inclined 
to think is the most important element, a dominant species, which 
has already beaten many competitors in its own home, will tend to 
spread and supplant many others. Alph. de CandoUe has shown 
that those species which spread widely, tend generally to spread 
very widely ; consequently, they will tend to supplant and exter- 
minate several species in several areas, and thus check the inordinate 
increase of specific forms throughout the world. Dr. Hooker has 
recently shown that in the' S.E. corner of Australia, where, appa- 
rently, there are many invaders from different quarters of the globe, 
the endemic Australian species have been greatly reduced in number. 
How much weight to attribute to these several considerations I 
will not pretend to say ; but conjointly they must limit in each 
country the tendency to an indefinite augmentation of specific 

Summary of Chaj)ter. 

C If under changing conditions of life organic beings present indivi- 
I dual differences in almost every part of their structure, and this 
i cannot be disputed ; if there be, owing to their geometrical rate of 
,. increase, a severe straggle for life at some age, season, or year, and 
this certainly cannot be disputed ; then, considering the infinite 
. complexity of the relations of all organic beings to each other and to 
\S I their conditions of life, causing an infinite diversity in structure, con- 
stitution, and habits, to be advantageous to them, it would be a most 
extraordinary fact if no variations had ever occurred useful to each 
l)eing's own welfare, in the same manner as so many variations have 
_occurred useful to man. But if variations useful to any organic 
being ever do occur, assuredly individuals thus characterised will 
have the best chance of being preserved in the struggle for life ; 
and from the strong principle of inheritance, these will tend to 
produce offspring similarly characterised. This principle of pre- 

Chap. IV.] SL'MMAR^. 103 

servation, or the survival of the fittest, I have called Natural 
Selection. It le,ads-to the improvement of each creature in relation, 
to its organic and inorganic conditions of life ; and consequently, in 
most caseSj,l}e regarded as an advance in organisation. 
Nevertheless, low and simple forms will long endure if well fitted for 
their simple conditions of life. . — ' 

Natural selection, on the principle of qualities being inherited at 
corresponding ages, can modify the egg, seed, or young, as easily 
as the adult. Amongst many animals, sexual selection will have 
given its aid to ordinary selection, by assuring to the most vigorous 
and best adapted males the greatest number of offspring. Sexual 
selection will also give characters useful to the males alone, in their 
struggles or rivalry with other males ; and these characters will be 
transmitted to one sex or to both sexes, according to the form of 
inheritance which prevails. 

Whether natural selection has really thus acted in adapting the 
various forms of life to their several conditions and stations, must 
be judged by the general tenor and balance of evidence given in the 
following chapters. But we have already seen how it entails 
extinction ; and how largely extinction has acted in the world's 
history, geology plainly declares. Natural selection, also, leads to 
divergence of character; for the more organic beings diverge in 
structure, habits, and constitution, by so much the more can a large 
number be supported on the same area, — of which we see proof by 
looking to the inhabitants of any small spot, and to the productions 
naturalised in foreign lands. Therefore, during the modification of 
the descendants of any one species, and during the incessant struggle 
of all species to increase in numbers, the more diversified the de- 
scendants become, the better will be their chance of success in the 
battle for life. Thus the small differences distinguishing varieties 
of the same species, steadily tend to increase, till they equal the 
greater differences between species of the same genus, or even of 
distinct genera. 

We have seen that it is the common, the widely-diffused, and 
widely-ranging species, belonging to the larger genera within each 
class, which vary most ; and these tend to transmit to their modified 
offspring that superiority which now makes them dominant in 
their own countries. Natural selection, as has just been remarked: 
leads to divergence of character and to much extinction of the less 
improved and intermediate forms of life. On these principles, the 
nature of the affinities, and the generally well-defined distinctions 
between the innumerable organic beings in each class throughout 


the world, may be explained. It is a truly wonderful fact — tlie 
wonder of which we are apt to overlook from familiarity — that all 
animals and all plants throughout all time and spa££_sliould_be 
related to each other in groups subordinate to groups^ in the manner 
which we everywhere behold — namely, varieties of the same species 
most closely related, species of the same genus less closely and 
unequally related, forming sections and sub-genera, species of 
distinct genera much less closely related, and genera related in 
different degrees, forming sub-families, families, orders, sub-classes, 
and classes. The several subordinate groups in any class cannot be 
ranked in a single file, but seem clustered round points, and these 
round other points, and so on in almost endless cycles. If species 
had been independently created, no explanation would have been 
possible of this kind of classification ; but it is explained through 
inheritance and the complex action of natural selection, entailing 
extinction and divergence of character, as we have seen illustrated 
in the diagram. 

The affinities of all the beings of the same class have sometimes 
been represented by a great tree. I believe this simile largely 
speaks the truth. The green and budding twigs may represent 
sxisting species; and those produced during former years may 
represent the long succession of extinct species. 'At each period of 
growth all the growing twigs have tried to branch out on all sides, 
and to overtop and kill the surrounding twigs and branches, in the 
same manner as species and groups of species have at all times 
overmastered other species in the great battle for life. The limbs 
divided into great branches, and these into lesser and lesser branches, 
were themselves once, when the tree was young, budding twigs.* 
and this connection of the former and present buds by ramifying 
branches may well represent the classification of all extinct and 
living species in groups subordinate to groups. Of the many twigs 
which flourished when the tree was a mere bush, only two or three, 
now grown into great branches, yet survive and bear the other 
branches ; so with the species which lived during long-past geolo- 
gical periods, very few have left living and modified descendants^ 
["'From the first growth of the tree, many a limb and branch has 
decayed and dropped off; and these fallen branches of various sizes 
may represent those whole orders, families, and genera which have 
now no living representatives, and which are known to us only in 
a fossil state. As we here and there see a thin straggling branch 
springing from a fork low down in a tree, and which by some 
chance has been favoured and is still alive on its summit, so W6 

Ceap. IV.l SUMMARY. 106 

occasionally see an animal like the Ornithorhynchus or Lepidosiren, 
which in some small degree connects by its affinities two large 
branches of life, and which has apparently been saved from fatal 
competition by having inhabited a protected station. As buds give 
rise by growth to fresh buds, and these, if vigorous, branch out and 
overtop on all sides many a feebler branch, so by generation I 
believe it has been with the great Tree of Life, which fills with its 
dead and broken branches the crust of the earth, and covers tho 
surface with its ever-branching and beautiful ramifications. 



Laws of Variation. 

Effects of changed conditions — Use and disuse, combined with natural 
selection ; organs of flight and of vision — Acclimatisation — Correlated 
variation — Compensation and economy of growth — False correlations 
— Multiple, rudimentary, and lowly organised structures variable — 
Parts developed in an unusual manner are highly variable : specific 
characters more variable than generic: secondary sexual characters 
variable — Species of the same genus vary in an analogous manner — 
Reversions to long-lost characters — Summary. 

I HAVE hitherto sometimes spoken as if the variations — so common 
and multiform with organic beings under domestication, and in a 
lesser degree with those under nature — were due to chance. This, 
of course, is a wholly incorrect expression, but it serves to acknow- 
ledge plainly our ignorance of the cause of each pariiciilar variaHon. 
Some authors believe it to be as much the function of the repro- 
(||;fiti2e_system to produce individual differences, or slight devfatiohs 
of structureT'as to make the child like its i^arents. But the fact of 
variations and monstrosities occurring much more frequently under 
domestication than under nature, and the greater variability of species 
having wide ranges than of those with restricted ranges, lead to 
the conclusion that variability is generally related to the condi- 
tions of life to which each species has been exposed during several 
successive generations. In the first chapter I attempted to show that 
changed conditions act in two ways, directly on the whole organisa=;_ 
tion or on certain parts alone, and indirectly through the reproductive 
system. In all cases there are two factors, the nature of the 
organism, which is much the most important of the two, and the 
nature of the conditions. The direct action of changed conditions 
leads to definite or indefinite results. In the latter case the organi- 
sation seems to becopie plastic, and we have much fluctuating vari- 
ability. In the former case the nature of the organism is such 
tliat it yields readily, when subjected to certain conditions, and all, 
or nearly all the individuals become modified in the same way. 

It is very difficult to decide how far changed conditions, such as 
of climate, food, &c., have acted in a definite manner. There is 


reason to believe that in the course of time the effects have "been 
greater than can be proved by clear evidence. But we may safely 
conclude that the innumerable complex co-adaptations of structure, 
which ■we see throughout nature between various organic beings, 
cannot be attributed simply to such action. In the following cases 
the conditions seem to have produced some slight definite effect : E. 
Forbes asserts that shells at their southern limit, and when living in 
shallow water, are more bri'ghtly coloured than those of the same 
species from further north or from a greater depth ; but this 
certainly does not always hold good. Mr. Gould believes that birds 
of the same species are more brightly coloured under a clear atmos- 
phere, than when living near the coast or on islands ; and Wollaston 
is convinced that residence near the sea affects the colours of insects. 
Moquin-Tandon gives a list of plants which, when growing near the 
sea-shore, have their leaves in some degree fleshy, though not else- 
where fleshy. These slightly varying organisms are interesting in 
as far as they present characters analogous to those possessed by the 
species which are confined to similar conditions. 

When a variation is of the slightest use to any being, we cannot tell 
how much to attribute to the accumulative action of natural selection, 
and how much to the definite action of the conditions of life. Thus, 
it is well known to furriers that animals of the same species have 
thicker and better fur the further north they live ; but who can tell 
how much of this difference may be due to the warmest-clad indivi- 
duals having been favoured and preserved during many generations, 
and how much to the action of the severe climate ? for it would 
appear that climate has some direct action on the hair of our domes- 
tic quadrupeds. 

Instances could be given of similar varieties being produced from 
the same species under external conditions of life as different as can 
well be conceived ; and, on the other hand, of dissimilar varieties 
being produced under apparently the same external conditions. 
Again, innumerable instances are known to every naturalist, of 
species keeping true, or not varying at all, although living under 
the most opposite climates. Such considerations as these incline me 
to lay less weight on the direct action of the surrounding con- 
ditions, than on a tendency to vary, due to causes of which we are 
quite ignorant. 

In one sense the conditions of life may be said, not only to cause 
variability, either directly or indirectly, but likewise to include 
natural selection ; for the conditions determine whether this or that 
variety shall survive. But when man is the selecting agent, we 
clearly see that the two elements of change are distinct ; variability 


is in some manner excited, but it is the will of man wliicli aocumu- 
lates the variations in certain directions ; and it is this latter agency 
which answers to the survival of the fittest under nature. 

Effects of the increased Use and Disuse of PartSy as contrdled 
hy Natural Selection. 

From the facts alluded to in the first chapter, I think there can 
be no doubt that use in our domestic animals has strengthened 
and enlarged certain parts, and disu-se diminished them ; and that 
such modifications are inherited. Under free nature, we have no 
standard of comparison, by which to judge of the effects of long- 
continued use or disuse, for we know not the parent-forms ; but 
many animals possess structures which can be best explained by 
the effects of disuse. As Professor Owen has remarked, there is no 
greater anomaly in nature than a bird that cannot fly ; yet there are 
several in this state. The logger-headed duck of South America 
can only flap along the surface of the water, and has its wings in 
nearly the same condition as the domestic Aylesbury duck : it is a 
remarkable fact that the young birds, according to Mr. Cunning- 
ham, can fly, while the adults have lost this power. As the larger 
ground-feeding birds seldom take flight except to escape danger, it 
is probable that the nearly wingless condition of several birds, now 
inhabiting or which lately inhabited several oceanic islands, tenanted 
by no beast of prey, has been caused by disuse. The ostrich indeed 
inhabits continents, and is exposed to danger from which it cannot 
escape by flight, but it can defend itself by kicking its enemies, as 
efficiently as many quadrupeds. We may believe that the proge- 
nitor of the ostrich genus had habits like those of the bustard, and 
that, as the size and weight of its body were increased during suc- 
cessive generations, its legs were used more, and its wings less; 
until they became incapable of flight. 

Kirby has remarked (and I have observed the same fact) that the 
anterior tarsi, or feet, of many male dung-feeding beetles are often 
broken off ; he examined seventeen specimens in his own collection, 
and not one had even a relic left. In the Onites apelles the tai*si 
are so habitually lost, that the insect has been described as not 
having them. In some other genera they are present, but in a 
rudimentary condition. In the Ateuchus or sacred beetle of the 
Egyptians, they are totally deficient. The evidence that accidental 
mutilations can be inherited is at present not decisive ; but the 
remarkable cases observed by Brown-S^quard in guinea-pigs, of the 
inherited effects of operations, should make us cautious in denying 


this tendency. Hence it will perhaps be safest to look at the entire 
absence of the anterior tarsi in Ateuchus, and their rudimentary 
condition in some other genera, not as cases of inherited mutilations, 
but as due to the effects of long-continued disuse ; for as many 
dmig-feeding beetles are generally found with their tarsi lost, this 
ninst happen early in life ; therefore the tarsi cannot be of much 
importance or be much used by these insects. 

In some cases we might easily put down to disuse modifications 
of structure which are wholly, or mainly, due to natural selection. 
Mr. Wollaston has discovered the remarkable fact that 200 beetles. 
OTit of the 550 species (but more are now known) inhabiting 
Madeira, are so far deficient in wings that they cannot fly; and 
that, of the twenty-nine endemic genera, no less than twenty- three 
have all their species in this condition! Several facts, — namely, 
that beetles in many parts of the world are frequently blown to sea 
and perish ; that the beetles in Madeira, as observed by Mr. 
"Wollaston, lie much concealed, until the wind lulls and the sun 
shines; that the proportion of wingless beetles is largei on the 
exposed Desertas than in Madeira itself ; and especially the extra- 
ordinary fact, so strongly insisted on by Mr. Woilaston, that certain 
large groups of beetles, elsewhere excessively numerous, which 
absolutely require the use of their wings, are here almost entirely 
absent; — these several considerations make me believe that the 
wingless condition of so many Madeira beetles is mainly due to 
the action of natural selection, combined probably with disuse. For 
during many successive generations each individual beetle which 
flew least, either from its wings having been ever so little less perfectly 
developed or from indolent habit, will have had the best chance of 
surviving from not being blown out to sea ; and, on the other hand, 
those beetles which most readily took to flight would oftenest have 
been blown to sea, and thus destroyed. 

The insects in Madeira which are not ground-feeders, and which, 
as certain flower-feeding coleoptera and lepidoptera, must habitually 
use their wings to gain their subsistence, have, as Mr. Wollaston 
suspects, their wings not at all reduced, but even enlarged. This is 
quite compatible with the action of natural selection. For when a 
new insect first arrived on the island, the tendency of natural 
selection to enlarge or to reduce the wings, would depend on 
whether a greater number of individuals were saved by successfully 
battling with the winds, or by giving up the attempt and rarely or 
never flying. As with mariners shipwrecked near a coast, it would 
have been better for the good swimmers if they had been able to 
swim still further, whereas it would have been better for the bad 


swimmers if they had not been able to swim at all and had stuck 
to the wreck. 

The eyes of moles and of some burrowing rodents are rudimentary 
in size, and in some cases are quite covered by skin and fur. This 
state of the eyes is probably due to gradual reduction from disuse, 
but aided perhaps by natural selection. In South America, a bur- 
rowing rodent, the tuco-tuco, or Ctenomys, is even more subter- 
ranean in its habits than the mole ; and I was assured by a 
Spaniard, who had often caught them, that they were frequently 
blind. One which I kept alive was certainly in this condition, the 
cause, as appeared on dissection, having been inflammation of the 
nictitating membrane. As frequent inflammation of the eyes must 
be injurious to any animal, and as eyes are certainly not necessary 
to animals having subterranean habits, a reduction in their size, 
with the adhesion of the eyelids and growth of fur over them, 
might in such case be an advantage ; and if so, natural selection 
would aid the effects of disuse. 

It is well known that several animals, belonging to the most 
different classes, which inhabit the caves of Carniola and of Ken- 
tucky, are blind. In some of the crabs the foot-stalk for the eye 
remains, though the eye is gone ; — the stand for the telescope is 
there, though the telescope with its glasses has been lost. As it is 
difficult to imagine that eyes, though useless, could be in any way 
injurious to animals living in darkness, their loss may be attributed 
to disuse. In one of the blind animals, namely, the cave-rat 
(Neotoma), two of which were captured by Professor Silliman at 
above half a mile distance from the mouth of the cave, and there- 
fore not in the profoundest depths, the eyes were lustrous and ol 
large size ; and these animals, as I am informed by Professor Silli- 
man, after having been exposed for about a month to a graduated 
light, acquired a dim perception of objects. 

It is difficult to imagine conditions of life more similar than 
deep limestone caverns under a nearly similar climate ; so that, in 
accordance with the old view of the blind animals having been 
separately created for the American and European caverns, very 
close similarity in their organisation and affinities might have been 
expected. This is certainly not the case if we look at the two 
whole faunas ; and with respect to the insects alone, Schiodte has 
remarked, " We are accordingly prevented from considering the 
entire phenomenon in any other light than something purely local, 
and the similarity which is exhibited in a few forms between the 
Mammoth cave (in Kentucky) and the caves in Carniola, otherwise 
than as a very plain expression of that analogy which sulosista 


generally between the fauna of Europe and of North America." 
On my view we must suppose that American animals, having in 
most cases ordinary powers of vision, slowly migrated by successive 
generations from the outer world into the deeper and deeper re- 
cesses of the Kentucky caves, as did European animals into the 
caves of Europe. We have some evidence of this gradation of 
habit ; for, as Schiodte remarks, " We accordingly look upon the 
subterranean faunas as small ramifications which have penetrated 
into the earth from the geographically limited faunas of the adja- 
cent tracts, and which, as they extended themselves into darkness, 
have been accommodated to surrounding circumstances. Animals 
not far remote from ordinary forms, prepare the transition from 
light to darkness. Next follow those that are constructed for twi- 
light ; and, last of all, those destined for total darkness, and whose 
formation is quite peculiar." These remarks of Schiodte's, it should 
be understood, apply not to the same, but to distinct species. By 
the time that an animal had reached, after numberless generations, 
the deepest recesses, disuse will on this view have more or less 
perfectly obliterated its eyes, and natural selection will often have 
effected other changes, such as an increase in the length of the 
antennae or palpi, as a compensation for blindness. Notwithstand- 
ing such modifications, we might expect still to see in the cave- 
animals of America, affinities to the other inhabitants of that con- 
tinent, and in those of Europe to the inhabitants of the European 
continent. And this is the case with some of the American cave- 
animals, as I hear from Professor Dana ; and some of the European 
cave-insects are very closely allied to those of the surrounding 
country. It would be difficult to give any rational explanation of 
Ihe affinities of the blind cave-animals to the other inhabitants 
of the two continents on the ordinary view of their independent 
creation. That several of the inhabitants of the caves of the Old 
and New Worlds should be closely related, we might expect from 
the well-known relationship of most of their other productions. As 
a blind species of Bathyscia is found in abundance on shady rocks 
far from caves, the loss of vision in the cave-species of this one 
genus has probably had no relation to its dark habitation ; for it 
is natural that an insect already deprived of vision should readily 
become adapted to dark caverns. Another blind genus (Anoph- 
thalmus) offers this remarkable peculiarity, that the species, as 
Mr. Murray observes, have not as yet been found anywhere except 
in caves ; yet those which inhabit the several caves of Europe and 
America are distinct ; but it is possible that the progenitors of these 
several species, whilst they were furnished with eyes, may formerly 

112 ACCLIMATISATipN. [Chap. V 


have ranged over both continents, and then have become extinct, 
excepting in their present secluded abodes. Far from feeling sur- 
prise that some of the cave-animals should be very anomalous, as 
Agassiz has remarked in regard to the blind fish, the Amblyopsis, 
and as is the case with the blind Proteus with reference to the 
reptiles of Europe, I am only surprised that more wrecks of ancient 
life have not been preserved, owing to the less severe competition to 
which the scanty inhabitants of these dark abodes will have been 


Habit is hereditary with plants, as in the period of flowering, in 
the time of sleep, in the amount of rain requisite for seeds to germi- 
nate, &c., and this leads me to say a few words on acclimatisation. 
As it is extremely common for distinct species belonging to the same 
genus to inhabit hot and cold countries, if it be true that all the 
species of the same genus are descended from a single parent-form, 
acclimatisation must be readily effected during a long course of 
descent. It is notorious that each species is adapted to the climate 
of its ov/n home : species from an arctic or even from a temperate 
region cannot endure a tropical climate, or conversely. So again, 
many succulent plants cannot endure a damp climate. But the 
degree of adaptation of species to the climates under which they 
live is often overi-ated. We may infer this from our frequent in- 
ability to predict whether or not an imported plant will endure our 
climate, and from the number of plants and animals brought from 
different countries which are here perfectly healthy. We have 
reason to believe that species in a state of nature are closely limited 
in their ranges by the competition of other organic beings quite as 
much as, or more than, by adaptation to particular climates. But 
whether or not this adaptation is in most cases very close, we have 
evidence with some few plants, of their becoming, to a certain 
extent, natm'ally habituated to different temperatures ; that is, they 
become acclimatised : thus the pines and rhododendrons, raised from 
seed collected by Dr. Hooker from the same species growing at 
different heights on the Himalaya, were found to possess in this 
country different constitutional powers of resisting cold. Mr. 
Thwaites informs me that he has observed similar facts in Ceylon 5 
analogous observations have been made by Mr. H. C. Watson on 
European species of plants brought from the Azores to England : 
and I could give other cases. In regard to animals, several authentic 
instances could be adduced of species having largely extended, 
within historical times, their range from warmer to cooler latitudes, 


and conversely ; but v;e do not positively know that these ':«,iumais 
were strictly adapted to their native climate, though in all orcinary 
cases we assume such to be the case ; nor do we know th^t the^ 
have subsequently become specially acclimatised to their new 
homes, so as to be better fitted for them than they were at first. 

As we may infer that our domestic animals were originally chosen 
by uncivilised man because they were useful and because they bred 
readily under confinement, and not because they were subsequently 
found capable of far-extended transportation, the common and ex- 
traordinary capacity in our domestic animals of not only withstand- 
ing the most different climates, but of being perfectly fertile (a far 
severer test) under them, may be used as an argument that a large 
proportion of other animals now in a state of nature could easily 
be brought to bear widely different climates. We must not, how- 
ever, push the foregoing argument too far, on account of the pro- 
bable origin of some of our domestic animals from several wild 
stocks ; the blood, for instance, of a tropical and arctic wolf may 
perhaps be mingled in our domestic breeds. The rat and mouse 
cannot be considered as domestic animals, but they have been trans- 
ported by man to many parts of the world, and now have a far 
wider range than any other rodent ; for they live under the cold 
climate of Faroe in the north and of the Falklands in the south, 
and on many an island in the torrid zones. Hence adaptation to 
any special climate may be looked at as a quality readily grafted on 
an innate wide flexibility of constitution, common to most animals. 
CJn~this view, the capacity of enduring the most different climates 
by man himself and by his domestic animals, and the fact of the 
extinct elephant and rhinoceros having formerly endured a glacial 
climate, whereas the living species are now all tropical or sub- 
tropical in their habits, ought not to be looked at as anomalies, but 
as examples of a very common flexibility of constitution, brought, 
under peculiar circumstances, into action. 

How much of the acclimatisation of species to any peculiar 
climate is due to mere habit, and how much to the natural selection 
of varieties having different innate constitutions, and how much to 
both means combined, is an obscure question. That habit or custom 
has some influence, I must believe, both from analogy and from the 
incessant advice given in agricultural works, even in the ancient 
Encyclopoedias of China, to be very cautious in transporting ani- 
mals from one district to another. And as it is not likely that man 
should have succeeded in selecting so many breeds and sub-breeds 
with constitutions specially fitted for their own districts, the result 
must, I think, be due to habit. On the other hand, natural selec- 


tion would inevitably tend to preserve those individuals which, were 
born with constitutions best adapted to any country which they 
inhabited. In treatises on many kinds of cultivated plants, certain 
varieties are said to withstand certain climates better than others ; 
this is strikingly shown in works on fruit-trees published in the 
United States, in which certain varieties are habitually recom- 
mended for the northern and others for the southern States ; and as 
most of these varieties are of recent origin, they cannot ov/e their 
constitutional differences to habit. The case of the Jerusalem 
artichoke, which is never propagated in England by seed, and of 
which consequently new varieties have not been produced, has even 
been advanced, as proving that acclimatisation cannot be effected, 
for it is now as tender as ever it was ! The case, also, of the kidney- 
bean has been often cited for a similar purpose, and with much 
greater weight; but until some one will sow, during a score of 
generations, his kidney-beans so early that a very large proportion 
are destroyed by frost, and then collect seed from the few survivors, 
with care to prevent accidental crosses, and then again get seed 
from these seedlings, with the same precautions, the experiment 
cannot be said to have been tried. Nor let it be supposed that 
differences in the constitution of seedling kidney-beans never 
appear, for an account has been published how much more hardy 
some seedlings are than others ; and of this fact I have myself 
observed striking instances. 

On the whole, we may conclude that habit, or use and disuse, 
have, in some cases, played a considerable part in the modification 
of the constitution and structure ; but that the effects have often 
been largely combined with, and sometimes overmastered by, the 
natural selection of innate variations. 

Correlated Variation. 

I mean by this expression that the whole organisation is so tied 
together during its growth and development, that when slight 
variations in any one part occur, and are accumulated through 
natural selection, other parts become modified. This is a very im- 
portant subject, mosFimpeTTectly understood, and no doubt wholly 
different classes of facts may be here easily confounded together. 
We shall presently see that simple inheritance often gives the false 
appearance of correlation. One of the most obvious real cases is, 
that variations of structure arising in the young or larvae naturally 
tend to affect the structure of the mature animal. The several 
parts of the body which are homologous, and which, at an early 
embryonic period, are identical in structure, and which are necss- 


Barily exposed to similar conditions, seem eminently liable to vary 
in a like manner : we see this in the right and left sides of the 
body varying in the same manner ; in the front and hind legs, and 
even in the jaws and limbs, varying together, for the lower jaw 
is believed by some anatomists to be homologous with the limbs. 
These tendencies, I do not doubt, may be mastered more or less 
completely by natural selection : thus a family of stags once existed 
with an antler only on one side ; and if this had been of any great 
use to the breed, it might probably have been rendered permanent 
by selection. 

Homologous parts, as has been remarked by some authors, tend 
to cohere ; this is often seen in monstrous plants : and nothing 
is more common than the union of homologous parts in normal 
structures, as in the union of the petals into a tube. Hard parts 
seem to affect the form of adjoining soft parts ; it is believed by 
some authors that with birds the diversity in the shape of the 
pelvis causes the remarkable diversity in the shape of their kidneys. 
Others believe that the shape of the pelvis in the human mother 
influences by pressure the shape of the head of the child. In 
snakes, according to Schlegel, the form of the body and the manner 
of swallowing determine the position and form of several of the 
most important viscera. 

The nature of the bond is frequently quite obscure. M. Is. 
Geoffroy St. Hilaire has forcibly remarked, that certain malcon- 
formations frequently, and that others rarely, co-exist, without our 
being able to assign any reason. What can be more singular than 
the relation in cats between complete whiteness and blue eyes with 
deafness, or between the tortoise-shell colour and the female sex ; 
or in pigeons between their feathered feet and skin betwixt the outer 
toes, or between the presence of more or less down on the young 
pigeon when first hatched, with the future colour of its plumage : 
or, again, the relation between the hair and teeth in the naked 
Turkish dog, though here no doubt homology comes into play? 
With respect to this latter case of correlation, I think it can hardly 
be accidental, that the two orders of mammals which are most 
abnormal in their dermal covering, viz., Cetacea (whales) and 
Edentata (armadilloes, scaly ant-eaters, &c.), are likewise on the 
v/hole the most abnormal in their teeth ; but there are so many 
exceptions to this rule, as Mr. Mivart has remarked, that it has 
little value. 

I know of no case better adapted to show the importance of the 
laws of correlation and variation, independently of utility and 
therefore of natural selection, than that of the difference between 

I 2 


the outer and inner flowers in some Compositous and Umbellifei-ous 
plants. Every one is familiar with the difference between the ray 
and central florets of, for instance, the daisy, and this difference is 
often accompanied with the partial or- complete abortion of the 
reproductive organs. But in some of these plants, the seeds also 
differ in shape and sculpture. These differences have sometimes 
been attributed to the pressure of the involucra on the florets, or 
to their mutual pressure, and the shape of the seeds in the ray- 
florets of some Compositce countenances this idea ; but with the 
Umbellifer^e, it is by no means, as Dr. Hooker informs me, the 
species with the densest heads which most frequently differ in 
their inner and outer flowers. It might have been thought that 
the development of the ray-petals by drawing nourishment from the 
reproductive organs causes their abortion ; but this can hardly be 
the sole cause, for in some Compositse the seeds of the outer and 
inner florets differ, without any difference in the corolla. Possibly 
these several differences may be connected with the different flow 
of nutriment towards the central and external flowers : we know, 
at least, that with irregular flowers, those nearest to the axis arc 
most subject to peloria, that is to become abnormally symmetrical. 
I may add, as an instance of this fact, and as a striking case of 
correlation, that in many pelargoniums, the two upper petals in 
the central flower of the truss often lose their patches of darker 
colour ; and when this occurs, the adherent nectary is quite aborted ; 
the central flower thus becoming peloric or regular. When the 
colour is absent from only one of the two upper petals, the nectary 
is not quite aborted but is much shortened. 

With respect to the development of the corolla, Sprengel's idea 
that the ray-florets serve to attract insects, whose agency is highly 
advantageous or necessary for the fertilisation of these plants, is 
highly probable ; and if so, natural selection may have come into play. 
But with respect to the seeds, it seems impossible that their differ- 
ences in shape, which are not always correlated with any difference 
in the corolla, can be in any way beneficial : yet in the Umbelli- 
fer£e these differences are of such apparent importance — the seeds 
being sometimes orthospermous in the exterior flowers and coelo- 
spermous in the central flowers, — that the elder De Candolle 
founded his main divisions in the order on such characters. Hence 
modifications of structure, viewed by systematists as of high value, 
may be wholly due to the laws of variation and correlation, without 
being, as far as we can jticlge, of the slightest service to the species. 

We may often falsely attribute to correlated variation structures 
which are common to whole groups of species, and which in tmlh 


arc simply due to inheritance ; for an ancient progenitor may have 
acquired through natural selection some one modification in struc- 
ture, and, after thousands of generations, some other and inde- 
pendent modification ; and these two modifications, having been 
transmitted to a whole group of descendants with diverse habits, 
would naturally be thought to be in some necessary manner cor- 
related. Some other correlations are apparently due to the manner 
in which natural selection can alone act. For instance, Alph. de 
Candolle has remarked that winged seeds are never found in fruits 
which do not open : I should explain this rule by the impossibility 
of seeds gradually becoming winged through natural selection, unless 
the capsules were open ; for in this case alone could the seeds, which 
were a little better adapted to be wafted by the wind, gain an 
advantage over others less well fitted for wide dispersal. 

Compensation and Economy of Growth. 

The elder Geoffrey and Goethe propounded, at about the same time 
their law of compensation or balancement of growth ; or, as Goethe 
expressed it, " in order to spend on one side, nature is forced to 
economise on the other side." I think this holds true to a certain 
extent with our domestic productions : if nourishment flows to one 
part or organ in excess, it rarely flows, at least in excess, to another 
part ; thus it is difficult to get a cow to give much milk and to 
fatten readily. The same varieties of the cabbage do not yield 
abundant and nutritious foliage and a copious supply of oil-bearing 
seeds. When the seeds in our fruits become atrophied, the fruit 
itself gains largely in size and quality. In our poultry, a large 
tuft of feathers on the head is generally accompanied by a diminished 
comb, and a large beard by diminished wattles. With species in 
a state of nature it can hardly be maintained that the law is of 
universal application; but many good observers, more especially 
botanists, believe in its truth. I will not, however, here give any 
instances, for I see hardly any way of distinguishing between tho 
effects, on the one hand, of a part being largely developed through 
natural selection and another and adjoining part being reduced by 
ihis same process or by disuse, and, on the other hand, the actual 
withdrawal of nutriment from one part owing to the excess of 
growth in another and adjoining part. 

I suspect, also, that some of the cases of compensation which 
have been advanced, and likewise some other facts, may be merged 
under a more general principle, namely, that natural selection is 
continually trying to economise every part of the organisation. If 
under changed conditions of life a structure, before useful, becomes 


less useful, its di minuti o n will be favoured, for it will_grofit tho__ 
individual nofto^ave . lts_ jiutriment wasted in building-up an 
useless structure. I can thus only understand a fact with which 
I was much struck when examining cirripedes, and of which many- 
analogous instances could be given: namely, that when a cirripede is 
parasitic within another cirripede and is thus protected, it loses more 
or less completely its own shell or carapace. This is the case with 
the male Ibla, and in a truly extraordinary manner with the Proteo- 
lepas : for the carapace in all other cirripedes consists of the three 
highly-important anterior segments of the head enormously deve- 
loped, and furnished with great nerves and muscles; but in the 
parasitic and protected Proteolepas, the whole anterior part ot 
the head is reduced to the merest rudiment attached to the bases 
of the prehensile antennae. Now the saving of a large and complex 
structure, when rendered superfluous, would be a decided advantage 
to each successive individual of the species ; for in the struggle for 
life to which every animal is exposed, each would have a better 
chance of supporting itself, by less nutriment being wasted. 

Thus, as I believe, natural selection will tend in the long run to 
reduce any part of the organisation, as soon as it becomes, through 
changed habits, superfluous, without by any means causing some 
nther part to be largely developed in a corresponding degree. And, 
£onversely, that natural selection may perfectly well succeed in 
largely developing an organ without requiring as a necessary com- 
pensation the reduction of some adjoining part. 

Multiple, Rudimentary, and Lowly-organised Structures are 


It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, 
both with varieties and species, that when any part or organ is 
repeated many times in the same individual (as the vertebras in 
snakes, and the stamens in polyandrous flowers) the number is 
variable ; whereas the same part or organ, when it occurs in lesser 
numbers, is constant. The same author as well as some botanists 
have further remarked that multiple parts are extremely liable to 
vary in structure. As " vegetative repetition," to use Prof. Owen's 
expression, is a sign of low organisation, the foregoing statements 
accord with the common opinion of naturalists, that beings which 
stand low in the scale of nature are more variable than those which 
are higher. I presume that lowness here means that the several 
parts of the organisation have been but little specialised for particular 
functions ; and as long as the same part has to perform diversified 
work, we can perhaps see why it should remain variable, that is, 


why natural selection should not lave preserved or rejected each 
little deviation of form so carefully as when the part has to serve 
for some one special purpose. In the same way that a knife which 
has to cut all sorts of things may be of almost any shape ; whilst 
a tool for some particular purpose must be of some particular 
chape. Natural selection, it should never be forgotten, can act 
solely thi'ough and for the advantage of each being. 

Kudimentary parts, as it is generally admitted, are apt to be 
highly variable. We shall have to recur to this subject ; and I will 
nere only add that their variability seems to result from their use- 
lessness, and consequently from natural selection having had no 
power to check deviations in their structure. 

JL Part developed in any Species in an extrawdinary degree or 
manner^ in comparison with the same Fart in allied Species^ 
tends to be highly variable. 

Several years ago I was much struck by a remark, to the above 
effect, made by Mr. Watorhouse. Professor Owen, also, seems to 
have come to a nearly similar conclusion. It is hopeless to attempt 
to convince any one of the truth of the above proposition without 
giving the long array of facts which I have collected, and which 
cannot possibly be here introduced. I can only state my conviction 
that it is a rule of high generality. I am aware of several causes of 
error, but I hope that I have made due allowance for them. It 
should be understood that the rule by no means applies to any 
part, however unusually developed, unless it be unusually developed 
in. one species or in a few species in comparison with the same part 
in many closely allied species. Thus, the wing of a bat is a most 
abnormal structure in the class of mammals ; but the rule would 
not apply here, because the whole group of bats possesses wings ; it 
would apply only if some one species had wings developed in a 
remarkable manner in comparison with the other species of the 
same genus. The rule applies very strongly in the case of secondary 
sexual characters, when displayed in any unusual manner. The term, 
secondary sexual characters, used by Hunter, relates to characters 
which are attached to one sex, but are not directly connected with the 
act of reproduction. The rule applies to males and females ; but more 
rarely to the females, as they seldom offer remarkable secondary sexual 
characters. The rule being so plainly applicable in the case of secondary 
sexual characters, may be due to the great variability of these charac- 
ters, whether or not displayed in any unusual manner — of which fact 1 
tliink there can be little doubt. But that our rule is not confined 
to secondary sexual characters is clearly shown in the casa of 


liermaplirodite cirripedes ; I particularly attended to Mr. Water- 
house's remark, whilst investigating this Order, and I am fully con- 
vinced that the rule almost always holds good. I shall, in a future 
work, give a list of all the more remarkable cases ; I will here give 
only one, as it illustrates the rule in its largest application. The 
opercular valves of sessile cirripedes (rock barnacles) arc, in every 
sense of the word, very important structures, and they differ 
extremely little even in distinct genera ; but in the several species 
of one genus, Pyrgoma, these valves present a marvellous amount 
of diversification ; the homologous valves in the different specie^ 
being sometimes wholly unlike in shape ; and the amount of varia- 
tion in the individuals of the same species is so great, that it is no 
exaggeration to state that the varieties of the same species differ 
more from each other in the characters derived from these impor- 
tant organs, than do the species belonging to other distinct genera. 

As with birds the individuals of the same species, inhabiting the 
same country, vary extremely little, I have particularly attended to 
them ; and the rule certainly seems to hold good in this class. I 
cannot make out that it applies to plants, and this would have 
seriously shaken my belief in its truth, had not the great vari- 
ability in plants made it particularly difficult to compare their 
relative degrees of variability. 

When we see any part or organ developed in a remarkable degree 
or manner in a species, the fair presumption is that it is of high 
importance to that species ; nevertheless it is in this case eminently 
liable to variation. Why should this be so ? On the view that 
each species has been independently created, with all its parts as 
we now see them, I can see no explanation. But on the view that 
groups of species are descended from some other species, and have 
been modified through natural selection, I think we can obtain 
some light. First let me make some jDreliminary remarks. If, in 
our domestic animals, any part or the whole animal be neglected, 
and no selection be applied, that part (for instance, the comb in the 
Dorking fowl) or the whole breed will cease to have a uniform 
character ; and the breed may be said to be degenerating. In 
rudimentary organs, and in those which have been but little 
specialised for any particular purpose, and perhaps in polymorphic 
groups, we see a nearly parallel case ; for in such cases natural selec- 
tion either has not or cannot have come into full play, and thus the 
organisation is left in a fluctuating condition. But what here more 
particularly concerns us is, that those points in our domestic 
animals, which at the present time are undergoing rapid change by 
continued selection, are also eminently liable to variation. Look at 


the individuals of the same breed of the pigeon, and see what a 
prodigious amount of difference there is in the beaks of tumblers, in 
the beaks and wattle of carriers, in the carriage and tail of fan tails, 
&c., these being the points now mainly attended to by English 
fanciers. Even in the same sub-breed, as in that of the short-faced 
tumbler, it is notoriously difiScult to breed nearly perfect birds, 
many departing widely from the standard. There may truly be 
said to be a constant struggle going on between, on the one hand 
the tendency to reversion to a less perfect state, as well as an innate 
tendency to new variations, and, on the other hand, the power ol 
steady selection to keep the breed true. In the long run selection 
gains the day, and we do not expect to fail so completely as to breed 
bird as coarse as a common tumbler pigeon from a good short-faced 
strain. But as long as selection is rapidly going on, much variability 
in the parts undergoing modification may always be expected. 

Now let us turn to nature. When a part has been developed in 
an extraordinary manner in any one species, compared with the 
other species of the same genus, we may conclude that this part has 
undergone an extraordinary amount of modification since the period 
when the several species branched off from the common progenitor 
of the genus. This period will seldom be remote in any extreme 
degree, as species rarely endure for more than one geological period. 
An extraordinary amount of modification implies an unusually 
large and long-continued amount of variability, which has con- 
tinually been accumulated by natural selection for the benefit of 
the species. But as the variability of the extraordinarily developed 
part or organ has been so great and long-continued within a period 
not excessively remote, we might, as a general rule, still expect to 
find more variability in such parts than in other parts of the 
organisation which have remained for a much longer period nearly 
constant. And this, I am convinced, is the case. That the struggle 
between natural selection on the one hand, and the tendency to 
reversion and variability on the other hand, will in the course of 
time cease ; and that the most abnormally developed organs may be 
made constant, I see no reason to doubt. Hence, when an organ, 
however abnormal it may be, has been transmitted in approximately 
the same condition to many modified descendants, as in the case of 
the wing of the bat, it must have existed, according to our theory, 
for an immense period in nearly the same state ; and thus it has 
come not to be more variable than any other structure. It is only 
in those cases in which the modification has been comparatively 
recent and extraordinarily great that we ought to find the generative 
variability J as it may be called, still present in a high degree. For 


in this case the variability will seldom as yet have "been fixed by 
the continued selection of the individuals varying in the required 
manner and degree, and by the continued rejection of those tending 
to revert to a former and less-modified condition. 

Specific Characters more Variable than Generic Characters. 

The principle discussed under the last heading may be applied to 
our present subject. It is notorious that specific characters are 
more variable than generic. To explain by a simple example what 
is meant : if in a large genus of plants some species had blue 
flowers and some had red, the colour would be only a specific 
character, and no one would be surprised at one of the blue species 
varying into red, or conversely ; but if all the species had blue 
flowers, the colour would become a gen-eric character, and its varia- 
tion would be a more unusual circumstance. I have chosen this 
example because the explanation which most naturalists would 
advance is not here applicable, namely, that specific characters are 
more variable than generic, because they are taken from parts of less 
physiological importance than those commonly used for classing 
genera, I believe this explanation is partly, yet only indirectly, 
true; I shall, however, have to return to this point in the chapter 
on Classification. It would be almost superfluous to adduce evidence 
in support of the statement, that ordinary specific characters are 
more variable than generic ; but with respect to important charac- 
ters, I have repeatedly noticed in works on natural history, that 
when an author remarks with surprise that some important organ 
or part, which is generally very constant throughout a large group 
of species, differs considerably in closely-allied species, it is often 
variable in the individuals of the same species. And this fact shows 
that a character, which is generally of generic value, when it sinks 
in value and becomes only of specific value, often becomes variable, 
though its physiological importance may remain the same. Some- 
thing of the same kind applies to monstrosities : at least Is. Geofiroy 
St. Hilaire apparently entertains no doubt, that the more an organ 
normally differs in the different species of the same group, the more 
subject it is to anomalies in the individuals. 

On the ordinary view of each species having been independently 
created, why should that part of the structure, which difi"ers from 
the same part in other independently-created species of the same 
genus, be more variable than those parts which are closely alike in 
the several species? I do not see that any explanation can be 
given. But on the view that species are only strongly marked and 
fixed varisties, we might expect often to find them still continuing 


to vary in those parts of their structure which have varied within a 
niDderately recent period, and which have thus come to dififer. Or 
to state the case in another manner : — the points in which all the 
species of a genus resemble each other, and in which they differ 
from allied genera, are called generic characters ; and these characters 
may be attributed to inheritance from a common progenitor, for it 
can rarely have happened that natural selection will have modified 
Hcveral distinct species, fitted to more or less widely-different habits, 
in exactly the same manner : and as these so-called generic charac- 
ters have been inherited from before the period when the several 
species first branched off from their common progenitor, and subse- 
quently have not varied or come to difier in any degree, or only in a 
slight degree, it is not probable that they should vary at the present 
day. On the other hand, the points in which species differ from 
other species of the same genus are called specific characters ; and as 
these specific characters have varied and come to differ since the 
period when the species branched off from a common progenitor, it 
is probable that they should still often be in some degree variable, — 
at least more variable than those parts of the organisation which 
have for a very long period remained constant. 

Secondary Sexual Characters Variable. — I think it will be ad- 
mitted by naturalists, without my entering on details, that 
secondary sexual characters are highly variable. It will also be 
admitted that species of the same group differ from each other more 
widely in their secondary sexual characters, than in other parts of 
their organisation : compare, for instance, the amount of difference 
between the males of gallinaceous birds, in which secondary sexual 
characters are strongly displayed, with the amount of difference 
between the females. The cause of the original variability of these 
characters is not manifest ; but we can see why they should not 
have been rendered as constant and uniform as others, for they are 
accumulated by sexual selection, which is less rigid in its action 
than ordinary selection, as it does not entail death, but only gives 
fewer offspring to the less favoured males. Whatever the cause may 
be of the variability of secondary sexual characters, as they are 
highly variable, sexual selection will have had a wide scope for 
action, and may thus have succeeded in giving to the species of the 
same group a greater amount of difference in these than in other 

It is a remarkable fact, that the secondary differences between 
the two sexes of the same species are generally displayed in the very 
same parts of the organisation in which the species of the same 
genus differ from each other. Of this fact I will givo m iUus- 


tration the two first instances which happen to stand on my list ; 
and as the differences in these cases are of a very unusual nature, the 
relation can hardly be accidental. The same number of joints in 
the tarsi is a character common to very large groups of beetles, but 
in the Engidae, as Westwood has remarked, the number varies 
greatly ; and the number likewise differs in the two sexes of the 
same species. Again in the fossorial hymenoptera, the neuration oi 
the wings is a character of the highest importance, because common 
to large groups ; but in certain genera the nem'ation differs in the 
different species, and likewise in the two sexes of the same species. 
Sir J. Lubbock has recently remarked, that several minute crusta- 
ceans ofier excellent illustrations of this law. *'In Pontella, for 
instance, the sexual characters are afforded mainly by the anterior 
antennse and by the fifth pair of legs : the specific differences also 
are principally given by these organs." This relation has a clear 
meaning on my view : I look at all the species of the same genus as 
having as certainly descended from a common progenitor, as have 
the two sexes of any one species. Consequently, whatever part of 
the structure of the common progenitor, or of its early descendants, 
became variable, variations of this part would, it is highly probable, 
be taken advantage of by natural and sexual selection, in order to fit 
the several species to their several places in the economy of nature, 
and likewise to fit the two sexes of the same species to each other, 
or to fit the males to struggle with other males for the possession of 
the females. 

Finally, then, I conclude that the greater variability of specific 
characters, or those which distinguish species from species, than of 
generic characters, or those which are possessed by all the species ; 
— that the frequent extreme variability of any part which is deve- 
loped in a species in an extraordinary manner in comparison with 
the same part in its congeners ; and the slight degree of variability 
in a part, however extraordinarily it may be developed, if it be 
common to a whole group of species ; — that the great variability of 
secondary sexual characters, and their great difference in closely 
allied species ; — that secondary sexual and ordinary specific differ- 
ences are generally displayed in the same parts of the organisation, 
— are all principles closely connected together. All being mainly 
due to the species of the same group being the descendants of 
a common progenitor, from whom they have inherited much in 
common, — to parts which have recently and largely varied being 
more hkely still to go on varying than parts which have long been 
inherited and have not varied — to natural selection having more or 


less completely, according to the lapse of time, overniastercd the 
tendency to reversion and to fm'ther variability, — to sexual selection 
being less rigid than ordinary selection, — and to variations in the 
same parts having been accumulated by natural and sexual selection, 
and having been thus adapted for secondary sexual, and for ordinary 

Distinct Species present analogous Variations, so that a Variety 
of one Species often assumes a Character proper to an allied 
Species, or reverts to some of the Characters of an early Progenitor. 
— These propositions will be most readily understood by looking to 
our domestic races. The most distinct breeds of the pigeon, in 
countries widely apart, present sub-varieties with reversed feathers 
on the head, and with feathers on the feet, — characters not possessed 
by the aboriginal rock-pigeon ; these then are analogous variations 
in two or more distinct races. The frequent presence of fourteen 
or even sixteen tail-feathers in the pouter may be considered as a 
variation representing the normal structure of another race, the 
fantail. I presume that no one will doubt that all such analogous 
variations are due to the several races of the pigeon having inherited 
from a common parent the same constitution and tendency to 
variation, when acted on by similar unknown influences. In the 
vegetable kingdom we have a case of analogous variation, in the 
enlarged stems, or as commonly called roots, of the Swedish turnip 
and Kuta baga, plants which several botanists rank as varieties 
produced by cultivation from a common parent : if this be not so, 
the case will then be one of analogous variation in two so-called 
distinct species ; and to these a third may be added, namely, the 
common turnip. According to the ordinary view of each species 
Having been independently created, we should have to attribute 
this similarity in the enlarged stems of these three plants, not to 
the vera causa of community of descent, and a consequent tendency 
to vary in a like manner, but to three separate yet closely related 
acts of creation. Many similar cases of analogous variation have 
been observed by Naudin in the great gourd-family, and by various 
authors in our cereals. Similar cases occurring with insects under 
natural conditions have lately been discussed with much abihty by 
Mr. Walsh, who has grouped them under his law of Equable 

With pigeons, however, we have another case, namely, the occa- 
sional appearance in all the breeds, of slaty-blue birds with two 
black bars on the wings, white loins, a bar at the end of the tail, 
with the outer feathers externally edged near theii* bases with 
white. As all these marks are characteristic of the parent rock* 


pigeon, I presume that no cne will doubt that this is a case of 
reversion, and not of a new yet analogous variation appearing in 
the several breeds. We may, I think, confidently come to this 
conclusion, because, as we have seen, these coloured marks are 
eminently liable to appear in the crossed offspring of two distinct 
and differently coloured breeds ; and in this case there is nothing 
in the external conditions of life to cause the reappearance of the 
slaty-blue, with the several marks, beyond the influence of the mere 
act of crossing on the laws of inheritance. 

No doubt it is a very surprising fact that characters should re- 
appear after having been lost for many, probably for hundreds of 
generations. But when a breed has been crossed only once by some 
other breed, the offspring occasionally show for many generations vi 
tendency to revert in character to the foreign breed — some say, for 
a dozen or even a score of generations. After twelve generations, 
the proportion of blood, to use a common expression, from one 
ancestor, is only 1 in 2048 ; and yet, as we see, it is generally 
believed that a tendency to reversion is retained by this remnant of 
foreign blood. In a breed which has not been crossed, but in which 
loth parents have lost some character which their progenitor pos- 
sessed, the tendency, whether strong or weak, to reproduce the lost 
character might, as was formerly remarked, for all that we can see 
to the contrary, be transmitted for almost any number of gener- 
ations. When a character which has been lost in a breed, reappears 
after a great number of generations, the most probable hypothesis 
is, not that one individual suddenly takes after an ancestor 
removed by some hundred generations, but that in each successive 
generation the character in question has been lying latent, and at 
last, under unknown favourable conditions, is developed. With the 
barb-pigeon, for instance, which very rarely produces a blue bird, it 
is probable that there is a latent tendency in each generation tc 
produce blue plumage. The abstract improbability of such a ten- 
dency being transmitted through a vast number of generations, is 
not greater than that of quite useless or rudimentary organs being 
similarly transmitted. A mere tendency to produce a rudiment is 
indeed sometimes thus inherited. 

As all the species of the same genus are supposed to be descended 
from a common progenitor, it might be expected that they would 
occasionally vary in an analogous manner ; so that the varieties of 
two or more species would resemble each other, or that a variety 
of one species would resemble ii: certain characters another and 
distinct species, — this other species being, according to our view, 
only a well-marked and permanent variety. But characters exclu- 

ckap. v.] analogous variations. 127 

sively due to analogous variation would probably be of an unim- 
portant nature, for the preservation of all functionally important 
characters will have been determined through natural selection, in 
accordance with the different habits of the species. It might 
further be expected that the species of the same genus would occa- 
sionally exhibit reversions to long lost characters. As, however, 
we do not know the common ancestor of any natural group, we 
cannot distinguish between reversionary and analogous characters. 
If, for instance, we did not know that the parent rock-pigeon was 
not feather-footed or turn-crowned, we could not have told, whether 
such characters in our domestic breeds were reversions or only 
analogous variations ; but we might have inferred that the blue 
colour was a case of reversion from the number of the markings, 
which are correlated with this tint, and which would not probably 
have all appeared together from simple variation. More especially 
we might have inferred this, from the blue colour and the several 
marks so often appearing when differently coloured breeds are 
crossed. Hence, although under nature it must generally be left 
doubtful, what cases are reversions to formerly existing characters, 
and what are new but analogous variations, yet we ought, on our 
theory, sometimes to find the varying offspring of a species assuming 
characters which are already present in other members of the same 
group. And this undoubtedly is the case. 

The difficulty in distinguishing variable species is largely due to 
the varieties mocking, as it were, other species of the same genus. 
A considerable catalogue, also, could be given of forms intermediate 
between two other forms, which themselves can only doubtfully bo 
ranked as species ; and this shows, unless all these closely allied 
forms be considered as independently created species, that they 
have in varying assumed some of the characters of the others. But 
the best evidence of analogous variations is afforded by parts or 
organs which are generally constant in character, but which occa- 
sionally vary so as to resemble, in some degree, the same part or 
organ in an allied species. I have collected a long list of such 
cases ; but here, as before, I lie under the great disadvantage of not 
being able to give them. I can only repeat that such cases cer- 
tainly occur, and seem to me very remarkable. 

I will, however, give one curious and complex case, not indeed as 
affecting any important character, but from occurring in several 
species of the same genus, partly under domestication and partly 
under nature. It is a case almost certainly of reversion. The ass 
sometimes has very distinct transverse bars on its legs, like those 
on the legs of the zebra : it has been asserted that these are pis inest 


in the foal, and, from inquiries which I have made, I beUeve this 
to be true. The stripe on the shoulder is sometimes double, and 
is "very variable in length and outline. A white ass, but not an 
albino, has been described without either spinal or shoulder stripe : 
and these stripes are sometimes very obscure, or actually quite lost, 
in dark-coloured asses. The koulan of Pallas is said to have been 
seen with a double shoulder-stripe. Mr. Blyth has seen a specimen 
of the hemionus with a distinct shoulder-stripe, though it properly 
has none ; and I have been informed by Colonel Poole that the 
foals of this species are generally striped on the legs, and faintly on 
the shoulder. The quagga, though so plainly barred like a zebra 
over the body, is without bars on the legs ; but Dr. Gray has figured 
one specimen with very distinct zebra-like bars on the hocks. 

With respect to the horse, I have collected cases in England of 
the spinal stripe in horses of the most distinct breeds, and of all 
colours : transverse bars on the legs are not rare in duns, mouse- 
duns, and in one instance in a chestnut : a faint shoulder-stripe may 
sometimes be seen in duns, and I have seen a trace in a bay horse. 
My son made a careful examination and sketch for me of a dun 
Belgian cart-horse with a double stripe on each shoulder and witK 
leg-stripes ; I have myself seen a dun Devonshire pony, and a 
small dun Welsh pony has been carefully described to me, both 
with three parallel stripes on each shoulder. 

In the north-west part of India the Kattywar breed of horses is 
so generally striped, that, as I hear from Colonel Poole, who exa- 
mined this breed for the Indian Government, a horse without stripes 
is not considered as purely-bred. The spine is always striped ; the 
legs are generally barred ; and the shoulder-stripe, which is some- 
times double and sometimes treble, is common; the side of the 
face, moreover, is sometimes striped. The stripes are often plainest 
•in the foal ; and sometimes quite disappear in old horses. Colonel 
Poole has seen both gray and bay Kattywar horses striped when 
first foaled. I have also reason to suspect, from information given 
me by Mr. W. W. Edwards, that with the English race-horse the 
spinal stripe is much commoner in the foal than in the full-grown 
animal. I have myself recently bred a foal from a bay mare (off- 
spring of a Turcoman horse and a Flemish mare) by a bay English 
race-horse ; this foal when a week old was marked on its hinder 
quarters and on its forehead with -numerous, very narrow, dark, 
zebra-like bars, and its legs were feebly striped : all the stripes soon 
disappeared completely. Without here entering on further details, 
I may state that I have collected cases of leg and shoulder stripes- 
in horses of very different breeds in various countries from Britaia tc 


Eastern China ; and from Norway in the north to the Malay Archi- 
pelago in the south. In all parts of the world these stripes occur 
far oftenest in duns and mouse-duns ; by the term dun a large range 
of colour is included, from one between brown and black to a close 
approach to cream-colour. 

I am aware that Colonel Hamilton Smith, who has written on 
this subject, believes that the several breeds of the horse are 
descended from several aboriginal species — one of which, the dun, 
was striped ; and that the above-described appearances are all due 
to ancient crosses with the dun stock. But this view may be safely 
rejected ; for it is highly improbable that the heavy Belgian cart- 
horse, Welsh ponies, Norwegian cobs, the lanky Kattywar race, &c., 
inhabiting the most distant parts of the world, should all have 
been crossed with one supposed aboriginal stock. 

Now let us turn to the effects of crossing the several species of 
the horse-genus. Eollin asserts, that the common mule from the 
ass and horse is particularly apt to have bars on its legs ; accord- 
ing to Mr. Gosse, in certain parts of the United States about nine 
out of ten mules have striped legs. I once saw a mule with its 
legs so much striped that any one might have thought that it was 
a hybrid-zebra ; and Mr. W. C. Martin, in his excellent treatise on 
the horse, has given a figure of a similar mule. In four coloured 
drawings, which I have seen, of hybrids between the ass and zebra, 
the legs were much more plainly barred than the rest of the body ; 
and in one of them there was a double shoulder-stripe. In Lord 
Morton's famous hybrid from a chestnut mare and male quagga, 
the hybrid, and even the pure offspring subsequently produced 
from the same mare by a black Arabian sire, were much more 
plainly barred across the legs than is even the pure quagga. 
Lastly, and this is another most remarkable case, a hybrid has been 
figured by Dr. Gray (and he informs me that he knows of a second 
case) from the ass and the hemionus ; and this hybrid, though the 
ass only occasionally has stripes on his legs and the hemionus has 
none and has not even a shoulder-stripe, nevertheless had all four 
legs barred, and had three short shoulder-stripes, like those on the 
dun Devonshire and Welsh ponies, and even had some zebra-like 
stripes on the sides of its face. With respect to this last fact, I was 
so convinced that not even a stripe of colour ajipears from what is 
commonly called chance, that I was led solely from the occurrence 
of the face-stripes on this hybrid from the ass and hemionus to ask 
Colonel Poole whether such face-stripes ever occurred in the emi- 
nently striped Kattywar breed of horses, and was, as we have seen, 
answered in the affirmative. 



\Vnai uow are we to say to these several facts ? We see several 
distinct species of the horse-genus becoming, by simple variation, 
striped on the legs like a zebra, or striped on the shoulders like an 
ass. In the horse we se<3 this tendency strong whenever a dun tint 
appears — a tint which approaches to that of the general colouring 
of the other speoics of the genus. The appearance of the stripes is not 
accompanied oy any change of form or by any other new character. 
We see this tendency to become striped most strongly displayed in 
hybrids from between several of the most distinct species. Now 
observe the case of the several breeds of pigeons : they are descended 
from a pigeon (including two or three sub-species or geographical 
races) of a bluish colour, with certain bars and other marks ; and when 
any breed assumes by simple variation a bluish tint, these bars and 
other marks invariably reappear ; but without any other change of 
form or character. When the oldest and truest breeds of various 
colours are crossed, we see a strong tendency for the blue tint and bare 
and marks to reappear in the mongrels. I have stated that the most 
probable hypothesis to account for the reappearance of very ancient 
characters, is — that there is a tendency in the young of each succes- 
sive generation to produce the long-lost character, and that this 
tendency, from imknown causes, sometimes prevails. And we have 
just seen that in several species of the horse-genus the stripes are 
either plainer or appear more commonly in the young than in the 
old. Call the breeds of pigeons, some of which have bred true for 
centuries, species ; and how exactly parallel is the case with that ot 
the species of the horse-genus ! For myself, I venture confidently 
to look back thousands on thousands of generations, and I see an 
animal striped like a zebra, but perhaps otherwise very differently 
constructed, the common parent of our domestic horse (whether or 
not it be descended from one or more wild stocks) of the ass, the 
hemionus, quagga, and zebra. 

He who believes that each equine species was independently 
created, will, I presume, assert that each species has been created 
with a tendency to vary, both under nature and under domestication, 
in this particular manner, so as often to become striped like the 
other species of the genus ; and that each has been created with 
a strong tendency, when cros.3ed with species inhabiting distant 
quarters of the world, to produce hybrids resembling in their 
stripes, not their own parents, but other species of the genus. To 
admit this view is, as it seems to me, to reject a real for an unreal, 
or at least for an unknown, cause. It makes the works of God a 
mere mockery and deception ; I would almost as soon believe with 
the old and ignorant cosmogonists, that fossil shells had never lived, 


but had been created in stone so as to mock the shells living on the 

Summary. — Our ignorance of the laws of variation is profound. 
Not in one case out of a hundred can we pretend to assign any 
reason why this or that part has varied. But whenever we have 
the means of instituting a comparision, the same laws appear to 
have acted in producing the lesser differences between varieties of 
the same species, and the greater differences between species of the 
same genus. Changed conditions generally induce mere fluctuating 
variability, but sometimes they cause direct and definite effects ; 
and these may become strongly marked in the course of tim?, 
though we have not sufficient evidence on this head. Habit in 
producing constitutional peculiarities and use in strengthening and 
disuse in weakening and diminishing organs, appear in many cases 
to have been potent in their effects. Homologous parts tend to vary 
in the same manner, and homologous parts tend to cohere. Modifi- 
cations in hard parts and in external parts sometimes affect softer 
and internal parts. When one part is largely developed, perhaps it 
tends to draw nourishment from the adjoining parts ; and every 
part of the structure which can be saved without detriment will be 
saved. Changes of structure at an early age may affect parts sub- 
sequently developed ; and many cases of correlated variation, the 
nature of which we are unable to understand, undoubtedly occur. 
Multiple parts are variable in number and in structure, perhaps 
arising from such parts not having been closely specialised for any 
particular function, so that their modifications have not been closely 
checked by natural selection. It follows probably from this same 
cause, that organic beings low in the scale are more variable than 
those standing higher in the scale, and which have their whole 
organisation more specialised. Rudimentary organs, from being 
useless, are not regulated by natural selection, and hence are 
variable. Specific characters — that is, the characters which have 
come to differ since the several species of the same genus branched 
off from a common parent — are more variable than generic cha- 
racters, or those which have long been inherited, and have not 
differed within this same period. In these remarks we have re- 
ferred to special parts or organs being still variable, because they 
have recently varied and thus come to differ ; but we have also seen 
in the second chapter that the same principle applies to the whole 
Individual ; for in a district where many species of a genus are 
found — that is, where there has been much former variation and 
differentiation, or where the manufactory of new specific forms has 
been actively at work — in that district and amongst these sj^ecies, 

K 2 


we now find, on an average, most varieties. Secondary sexua] 
characters arc highly variable, and such characters difier much in 
the species of the same group. Variability in the same parts of the 
organisation has generally been taken advantage of in giving secon- 
dary sexual diiferences to the two sexes of the same species, and 
specific differences to the several species of the same genus. Any 
part or organ developed to an extraordinary size or in an extra- 
ordinary manner, in comparison with the same part or organ in the 
allied species, must have gone through an extraordinary amount of 
modification since the genus arose; and thus we can understand 
why it should often still be variable in a much higher degree than 
other parts ; for variation is a long-continued and slow process, and 
natural selection will in such cases not as yet have had time to 
overcome the tendency to further variability and to reversion to a 
less modified state. But when a species with any extraordinarily- 
developed organ has become the parent of many modified descen- 
dants — which on our view must be a very slow process, requiring a 
long lapse of time — in this case, natural selection has succeeded in 
giving a fi.xed character to the organ, in however extraordinary a 
manner it may have been developed. Species inheriting nearly the 
same constitution from a common parent, and exposed to similar 
influences, naturally tend to present analogous variations, or these 
same species may occasionally revert to some of the characters of 
their ancient progenitors. Although new and important modifica- 
tions may not arise from reversion and analogous variation, such 
modifications will add to the beautiful and harmonious diversity of 

Whatever the cause may be of each slight difference between the 
offspring and their parents — and a cause for each must exist — ^we 
have reason to believe that it is the steady accumulation of bene- 
ficial diflferences which has given rise to all the more important 
modifications of structure in relation to tbe habits of er-ch Rpfxiiea. 



Difficulties of the Theory . 

Difficulties of the theory of descent with modification — Absence or rarity 
of transitional varieties — Transitions in habits of life — Diversified 
habits in the same species — Species with habits widely different 
from these of their allies — Organs of extreme perfection — Modes of 
transition — Cases of difficulty — Natura non focit saltum — Organs 
of small importance — Organs not in all cases absolutely perfect — 
The law of Unity of Type and of the Conditions of Existence embraced 
by the theory of Natural Selection. 

Long before the reader has arrived at this part of my work, a crowd 
of difficulties will have occurred to him. Some of them are so 
serious that to this day I can hardly reflect on them without being 
in some degree staggered ; but, to the best of my judgment, the 
greater number are only apparent, and those that are real are not, 
1 think, fatal to the theory. 

These difficulties and objections may be classed under the follow- 
ing heads : — First, why, if species have descended from other species 
by fine gradations, do we not everywhere see innumerable tran- 
sitional forms ? Why is not all nature in confusion, instead of the 
species being, as we see them, well defined ? 

Secondly, is it possible that an animal having, for instance, the 
structure and habits of a bat, could have been formed by the modifi- 
cation of some other animal with widely-different habits and 
structure? Can we believe that natural selection could produce 
on the one hand, an organ of tiifling importance, such as the tail of 
a giraffe, which serves as a fly-flapper, and, on the other hand, ac 
organ so wonderful as the eye ? 

Thirdly, can instincts be acquired and modified through natural 
I Belection ? What shall we say to the instinct which leads the bee / 
to make cells, and which has practically anticipated the discoveries / 
of profound mathematicians ? 

Fourthly, how can we account for species, when crossed, being 
sterile and producing sterile offspring, whereas, when varieties are 
srossed, their fertility is unimpaired ? 

The two first heads will here be discussed ; some uiiscellaneou3 


objections in the following chapter ; Instinct and Hybridisni in the 
two succeeding chapters. 

On the Absence or Rarity of Transitional Varieties. — As natural 
selection acts solely by the preservation of profitable modifications, 
each new form will tend in a fully-stocked country to take the place 
of, and finally to exterminate, its own less improved parent-form 
and other less-favoured forms with which it comes into competition. 
Thus extinction and natural selection go hand in hand. Hence, if 
we look at each species as descended from some unknown form, 
both the parent and all the transitional varieties will generally have 
been exterminated by the very process of the formation and }M3r- 
fection of the new form. 

But, as by this theory innumerable transitional forms must have 
existed, why do we not find them embedded in countless numbers 
in the crust of the earth ? It will be more convenient to discuss 
this question in the chapter on the Imperfection of the Geological 
Kecord ; and I will here only state that I believe the answer mainly 
lies in the record being incomparably less perfect than is generally 
supposed. The crust of the earth is a vast museum ; but the 
natural collections have been imperfectly made, and only at long 
intervals of time. 

But it may be urged that when several closely-allied species 
inhabit the same territory, we surely oaght to find at the present 
time many transitional forms. Let us take a simple case : in 
i ravelling from north to south over a continent, we generally meet 
at successive intervale with closely allied or representative species, 
evidently filling nearly the same place in the natural economy of 
the land. These representative species often meet and interlock ; 
and as the one becomes rarer and rarer, the other becomes more and 
more frequent, till the one replaces the other. But if we compare 
these species where they intermingle, they are generally as absolutely 
distinct from each other in every detail of structure as are specimens 
taken from the metropolis inhabited by each. By my theory these 
allied species are descended from a common parent ; and during the 
process of modification, each has become adapted to the conditions 
of life of its own region, and has supplanted and exterminated its 
original parent-form and all the transitional varieties between its 
past and present states. Hence we ought not to expect at the 
present time to meet with numerous transitional varieties in each 
region, though they must have existed there, and may be embedded 
there in a fossil condition. But in the intermediate region, having 
intermediate conditions of life, why do we not now find closely- 
linking intermediate varieties? This difllculty for a long time 


quite confounded me. But I think it can be in large part ex- 

In the first place we should be extremely cautious in inferring, 
because an area is now continuous, that it has been continuous 
during a long period. Geology would lead us to believe that most 
continents have been broken up into islands even during the later 
tertiary periods ; and in such islands distinct species might have 
been separately formed without the possibility of inteniicdiate 
varieties existing in the intermediate zones. By changes in the 
form of the land and of climate, marine areas now continuous must 
often have existed within recent times in a far less continuous and 
uniform condition than at present. But I will pass over this way 
of escaping from the difficulty ; for I believe that many perfectly 
defined species have been formed on strictly continuous areas ; 
though I do not doubt that the formerly broken condition of areas 
now continuous, has played an important part in the formation of new 
species, more especially with freely-crossing and wandering animals. 

in looking at species as they are now distributed over a wide 
area, we generally find them tolerably numerous over a large 
territory, then becoming somewhat abruptly rarer and rarer on the 
confines, and finally disappearing. Hence the neutral territory 
between two representative species is generally narrow in comparison 
with the territory proper to each. We see the same fact in 
ascending mountains, and sometimes it is quite remarkable how 
abruptly, as Alph. de Candolle has observed, a common alpine 
species disappears. The same fact has been noticed by E. Forbes 
in sounding the depths of the sea with the dredge. To those who 
look at climate and the physical conditions of life as the all- 
important elements of distribution, these facts ought to cause 
surprise, as climate and height or depth graduate away insensibly. 
But when we bear in mind that almost every species, even in its 
metropolis, would increase immensely in numbers, were it not for 
other competing species ; that nearly all either prey on or serve as 
prey for others ; in short, that eacn organic being is either directly 
or indirectly related in the most important manner to other organic 
beings, — we see that the range of the inhabitants of any country 
by no means exclusively depends on insensibly changing physical 
conditions, but in a large part on the presence of other species, on 
which it lives, or by which it is destroyed, or with which it comes 
into competition ; and as these species are already defined objects, 
not blending one into another by insensible gradations, the range of 
any one species, depending as it does on the range of others, will 
tend to be sharply defined. Moreover, each species on the confines 

130 ABSENCE OR RARITY [Chap.\1. 

of its range, where it exists in lessened numbers, will, during fluctu- 
ations in the number of its enemies or of its prey, or in the nature 
of the seasons, be extremely liable to utter extermination ; and thus 
its geographical range will come to be still more sharply defined. 

As allied or representative species, when inhabiting a continuous 
area, are generally distributed in such a manner that each has a 
wide range, with a comparatively narrow neutral territory between 
them, in which they become rather suddenly rarer and rarer ; then, 
as varieties do not essentially differ from species, the same rule will 
probably apply to both ; and if we take a varying species inhabiting 
a very large area, we shall have to adapt two varieties to two large 
areas, and a third variety to a narrow intermediate zone. The 
intermediate variety, consequently, will exist in lesser numbers 
from inhabiting a narrow and lesser area ; and practically, as far as 
I can make out, this rule holds good with varieties in a state of 
nature. I have met with striking instances of the rule in the case 
of varieties intermediate between well-marked varieties in the genus 
Balanus. And it would appear from information given me by Mr. 
Watson, Dr. Asa Gray, and Mr. Wollaston, that generally, when 
varieties intermediate between two other forms occur, they are much 
rarer numerically than the forms which they connect. Now, if we 
may trust these facts and inferences, and conclude that varieties 
linking two other varieties together generally have existed in lesser 
numbers than the forms which they connect, then we can understand 
why intermediate varieties should not endure for very long periods : 
— why, as a general rule, they should be exterminated and disappear, 
fiooner than the forms which they originally linked together. 

For any form existing in ksser numbers would, as already 
remarked, run a greater chance of being exterminated than one 
existing in large numbers ; and in this particular case the inter- 
mediate form would be eminently liable to the inroads of closely- 
allied forms existing on both sides of it. But it is a far more 
important consideration, that during the process of further modifi- 
cation, by which two varieties are supposed to be converted and 
perfected into two distinct species, the two which exist in larger 
numbers, from inhabiting larger areas, will have a great advantage 
over the intermediate variety, which exists in smaller numbers 
in a narrow and intermediate zone. For forms existing in larger 
numbers will have a better chance, within any given period, of 
presenting further favourable variations for natural selection to 
seize on, than will the rarer forms which exist in lesser numbers. 
ITence, the moT3 common forms, in the race for life, will tend to 
beat and supplant the less common forms, for these wuU be mora 


slowly modified and improved. It is the same principle which, as 
I believe, accounts for the common species in each coimtry, as 
shown in the second chapter, presenting on an average a greater 
number of well-marked varieties than do the rarer species. I may 
illustrate what I mean by supposing three varieties of sheep to be 
kept, one adapted to an extensive mountainous region ; a second 
to a comparatively narrow, hilly tract; and a third to the wide 
plains at the base ; and that the inhabitants are all trying with 
equal steadiness and skill to improve their stocks by selection ; the 
chances in this case will be strongly in favour of the great holders 
on the mountains or on the plains, improving their breeds more 
quickly than the small holders on the intermediate narrow, hilly 
tract ; and consequently the improved mountain or plain breed will 
soon take the place of the less improved hill breed ; and thus the 
two breeds, which originally existed in greater numbers, will come 
into close contact with each other, without the interposition of the 
supplanted, intermediate hill- variety. 

To sum up, I believe that species come to be tolerably well- 
defined objects, and do not at any one period present an inextricable 
chaos of varying and intermediate links : first, because new varie- 
ties are very slowly formed, for variation is a slow process, and 
natural selection can do nothing until favourable individual 
differences or variations occur, and until a place in the natural 
polity of the country can be better filled by some modification of 
some one or more of its inhabitants. And such new places will 
depend on slow changes of climate, or on the occasional immigration 
of new inhabitants, and, probably, in a still more important degree, 
on some of the old inhabitants becoming slowly modified, with the 
new forms thus produced and the old ones acting and reacting on 
each other. So that, in any one region and at any one time, we 
ought to see only a few species presenting slight modifications of 
structure in some degree permanent ; and this assuredly we do see. 

Secondly, areas now continuous must often have existed within 
the recent period as isolated portions, in which many forms, more 
especially amongst the classes which unite for each birth and 
wander much, may have separately been rendered sufficiently 
distinct to rank as representative species. In this case, inter- 
mediate varieties between the several representative species and 
their common parent, must formerly have existed within each 
isolated portion of the land, but these links during the process of 
natural selection will have been supplanted and exterminated, so 
that they will no longer be found in a living state. 

Thirdly, when two or more varieties have been formed in different 


portions of a strictly continuous area, intermediate varieties will, it 
is probable, at first have been formed in the intermediate zones, but 
they will generally have had a short duration. For these inter- 
mediate varieties will, frcm reasons already assigned (namely from 
what we know of the actual distribution of closely allied or repre- 
sentative species, and likewise of acknowledged varieties), exist in 
the intermediate zones in lesser numbers than the varieties which 
they tend to connect. From this cause alone the intermediate 
varieties will bo liable to accidental extermination ; and during the 
process of further modification through natural selection, they will 
almost certainly be beaten and supplanted by the forms which they 
connect ; for these from existing in greater numbers will, in the 
aggregate, present more varieties, and thus be further improved 
through natural selection and gain further advantages. 

Lastly, looking not to any one time, but to all time, if my theory 
be true, numberless intermediate varieties, linking closely together 
all the species of the same group, must assuredly have existed ; but 
the very process of natural selection constantly tends, as has been. 
60 often remarked, to exterminate the parent-forms and the inter- 
mediate links. Consequently evidence of their former existence 
could be found only amongst fossil remains, which are preserved, as 
we shall attempt to show in a future chapter, in an extremely im- 
perfect and intermittent record. 

071 the Origin and Transitions of Organic Beings with ^peculiar 
Ilahits and Structure. — It has been asked by the opponents of such 
views as I hold, how, for instance, could a land carnivorous animal 
have been converted into one with aquatic habits ; for how could 
the animal in its transitional state have subsisted ? It would be 
easy to show that there now exist carnivorous animals presenting 
close intermediate grades from strictly terrestrial to aquatic habits : 
and as each exists by a struggle for life, it is clear that each must be 
well adapted to its place in nature. Look at the Mustela vison of 
North America, which has webbed feet, and which resembles an 
otter in its fur, short legs, and form of tail. During the summer 
this animal dives for and preys on fish, but during the long winter 
it leaves the frozen waters, and preys, like other pole-cats, on mice 
and land animals. If a different case had been taken, and it had 
been asked how an insectivorous quadruped could possibly have been 
converted into a flying bat, the question would have been far more 
difficult to answer. Yet I think such difficulties have little weight. 

Here, as on other occasions, I lie under a heavy disadvantage, for, 
out of the many striking case? which I have collected, I can give 
only one or two instances of transitional habits and st<-ucturc8 in 


allied species ; and of diversified habits, eitlier constant or occa- 
Bional, in the same species. And it seems to me that nothing less 
than a long list of such cases is sufficient to lessen the difficulty in 
any particular case like that of the bat. 

Look at the family of squirrels ; here we have the finest gra 
dation from animals with their tails only slightly flattened, and 
from others, as Sir J. Richardson has remarked, with the posterior 
part of their bodies rather wide and with the skin on their flanks 
rather full, to the so-called flying squirrels ; and flying squirrels 
have their limbs and even the base of the tail united by a broad 
expanse^ skin, which serves as ajQarachute and allows tliem to 
glide through the air to an astonishing distance from tree to tree. 
We cannot doubt that each structure is of use to each kind of 
squirrel in its own country, by enabling it to escape birds or beasts 
of prey, to collect food more quickly, or, as there is reason to 
believe, to lessen the danger from occasional falls. But it does not 
follow from this fact that the structure of each squirrel is the best 
that it is possible to conceive under all possible conditions. Let 
the climate and vegetation change, let other competing rodents or 
new beasts of prey immigrate, or old ones become modified, and all 
analogy would lead us to believe that some at least of the squirrels 
would decrease in numbers or become exterminated, unless they 
also became modified and improved in structure in a corresponding 
manner. Therefore, I can see no difficulty, more especially under 
changing conditions of life, in the continued preservation of indi- 
viduals with fuller and fuller flank-membranes, each modification 
being useful, each being propagated, until, by the accumulated 
effects of this process of natural selection, a perfect so-called flying 
squirrel was produced. 

Now look at the Galeopithecus or so-called flying lemur, which 
formerly was ranked amongst bats, but is now believed to belong 
to the Insectivora. An extremely wide flank-membrane stretches 
from the corners of the jaw to the tail, and includes the limbs 
with the elongated fingers. This flank-membrane is furnished with 
an extensor muscle. Although no graduated links of structure, fitted 
for gliding through the air, now connect the Galeopithecus with 
the other Insectivora, yet there is no difficulty in supposing that 
such links formerly existed, and that each was developed in the 
same manner as with the less perfectly gliding squirrels ; each grade 
of structure having been useful to its possessor. Nor can I sue 
any insuperable diffictilty in further believing that the membrune 
connected fingers and fore-arm of the Galeopithecus might have 
been greatly lengthened by natural selection ; and this, as far as t!i« 


organs of flight are concerned, ssrould have converted the animal 
into a bat. In certain bats in which the wing-membrane extends 
from the top of the shoulder to the tail and includes the hind-legs, 
we perhaps see traces of an apparatus originally fitted for gliding 
through the air rather than for flight. 

If about a dozen genera of birds were to become extinct, who 
would have ventured to surmise that birds might have existed 
which used their wings solely as flappers, like the logger-headed 
duck (Micropterus of Eyton) ; as fins in the water and as front-legs 
on the land, like the penguin ; as sails, Uke the ostrich ; and func- 
tionally for no purpose, like the Apteryx ? Yet the structure of 
each of these birds is good for it, under the conditions of life to 
which it is exposed, for each has to live by a struggle ; but it is not 
necesarily the best possible under all possible conditions. It must 
not be inferred from these remarks that any of the grades of wing- 
structure here alluded to, which perhaps may all be the result of 
disuse, indicate the steps by which birds actually acquired their 
perfect power of flight; but they serve to show what diversified 
means of transition are at least possible. 

Seeing that a few members of such v/ater-breathing classes as 
the Crustacea and Mollusca are adapted to live on the land ; and 
seeing that we have flying birds and mammals, flying insects of the 
most diversified types, and formerly had flying reptiles, it is con- 
ceivable that flying-fish, which now glide far through the air, 
slightly rising and turning by the aid of their fluttering fins, might 
have been modified into perfectly winged animals. If this had 
been effected, who would have ever imagined that in an early 
transitional state they had been the inhabitants of the open ocean, 
and had used their incipient organs of flight exclusively, as far as 
we know, to escape being devoured by other fish ? 

When we see any structure highly perfected for any particular 
habit, as the wings of a bird for flight, we should bear in mind that 
animals displaying early transitional grades of the structure \\i\] 
seldom have survived to the present day, for they will have been 
supplanted by their successors, which were gradually rendered more 
perfect through natural selection. Furthermore, we may conclude 
that transitional states between structures fltted for very different 
habits of life will rarely have been developed at an early period in 
great numbers and under many subordinate forms. Thus, to return 
to our imaginary illustration of the flying-fish, it does not seem 
probable that fishes capable of true flight would have been developed 
under many subordinate forms, for taking prey of many kinds in 
many ways, on the land and in the water, until their organs of flight 


had come to a high stage of perfection, so as to have given them a 
decided advantage over other animals in the battle for life. Hence 
the chance of discovering species with transitional grades of struc- 
ture in a fossil condition will always be less, from their having 
existed in lesser numbers, than in the case of species with fully 
developed structures. 

I will now give two or three instances both of diversified and of 
changed habits in the individuals of the same species. In either 
case it would be easy for natural selection to adapt the structure 
of the animal to its changed habits, or exclusively to one of its 
several habits. It is, however, difficult to decide, and immaterial 
for us, whether habits generally change first and structure after- 
wards ; or whether slight modifications of structure lead to clianged 
habits ; both probably often occurring almost simultaneously. Of 
cases of changed habits it will suffice merely to allude to that of the 
many British insects which now feed on exotic plants, or exclu- 
sively on artificial substances. Of diversified habits innumerable 
instances could be given : I have often watched a tyrant flycatcher 
(Saurophagus sulphuratus) in South America, hovering over one 
spot and then proceeding to another, like a kestrel, and at other 
times standing stationary on the margin of water, and then dashing 
into it like a kingfisher at a fish. In our own country the larger 
titmouse (Parus major) may be seen climbing branches, almost like 
a creeper ; it sometimes, like a shrike, kills small birds by blows 
on the head ; and I have many times seen and heard it hammering 
the seeds of the yew on a branch, and thus breaking them like a 
nuthatch. In North America the black bear was seen by Hearne 
swimming for hours with widely open mouth, thus catching, almost 
like a whale, insects in the water. 

As we sometimes see individuals following habits different from 
those proper to their species and to the other species of the same 
genus, we might expect that such individuals would occasionally 
give rise to new species, having anomalous habits, and with their 
structure either slightly or considerably modified from that of 
their type. And such instances occur in nature. Can a more 
striking instance of adaptation be given than that of a woodpecker 
for climbing trees and seizing insects in the chinks of the bark ? Yet 
in North America there are woodpeckers which feed largely on fruit, 
and others with elongated wings which chase insects on the wing. 
On the plains of La Plata, where hardly a tree grows, there is a 
woodpecker (Colaptes campestris) which has two toes before and 
two behind, a long pointed tongue, pointed tail-feathers, sufficiently 
stiff to support the bird in a vertical position on a post, but not so 


stiff as in the typical woodpeckers, and a straight strong beak. The 
beak, however, is not so straight or so strong as in the typical 
woodpeckers, but it is strong enough to bore into wood. Hence 
this Colaptes in all the essential parts of its structure is a vrood- 
pecker. Even in such trifling characters as the colouring, the 
harsh tone of the voice, and undulatory Aight, its close blood- 
relationship to our common woodpecker is plainly declared; yet, 
as I can assert, not only from my own observations, but from those 
of the accurate Azara, in certain large districts it does not climb 
trees, and it makes its nest in holes in banks ! In certain other 
districts, however, this same woodpecker, as Mr. Hudson states, 
frequents trees, and bores holes in the trunk for its nest. I may 
mention as another illustration of the varied habits of this genus, 
that a Mexican Colaptes has been described by De Saussure as 
boring holes into hard wood in order to lay up a store of acorns. 

Petrels are the most aerial and oceanic of birds, but in the quiet 
sounds of Tierra del Fuego, the Puffinuria berardi, in its general 
habits, in its astonishing power of diving, in its manner of swim- 
ming and of flying when made to take flight, would be mistaken 
by any one for an auk or a grebe ; nevertheless it is essentially a 
petrel, but with many parts of its organisation profoundly modified 
in relation to its new habits of life ; whereas the woodpecker ot 
La Plata has had its structure only slightly modified. In the case 
of the water-ouzel, the acutest observer by examining its dead body 
would never have suspected its sub-aquatic habits ; yet this bird, 
which is allied to the thrush family, subsists by diving — using its 
wings under water, and grasping stones with its feet. All the 
members of the great order of llymenopterous insects are terrestrial, 
excepting the genus Proctotrupes, which Sir John Lubbock has 
discovered to be aquatic in its habits ; it often enters the water and 
dives about by the use not of its legs but of its wings, and remains 
as long as four hours beneath the surface ; yet it exhibits no modi- 
fication in structure in accordance with its abnormal habits. 

He who believes that each being has been created as we now see 
it, must occasionally have felt surprise when he has met with an 
animal having habits and structure not in agreement. What can 
be plainer than that the webbed feet of ducks and geese are formed 
for swimming ? Yet there are upland geese with webbed feet which 
rarely go near the water ; and no one except Audubon has seen the 
frigate-bird, which has all its four toes webbed, alight on the surface 
of the ocean. On the other hand, grebes and coots are eminently 
aquatic, although their toes are only bordered by membrane. What 
seems plainer than that the long toes, not furnished with membrane 


of the Grallatores are formed for walking over swamps and floating 
plants? — the water-hen and landrail are members of this order, 
yet the first is nearly as aquatic as the coot, and the second nearly 
as terrestrial as the quail or partridge. In such cases, and many 
others could be given, habits have changed without a corresponding 
change of structure. The webbed feet of the upland goose may be 
said to have become almost rudimentary in function, though not 
in structure. In the frigate-bird, the deeply scooped membrane 
between the toes shows that structure has begun to change. 

He who believes in separate and innumerable acts of creation 
may say, that in these cases it has pleased the Creator to cause a 
being of one type to take the place of one belonging to another 
type ; but this seems to me only re-stating the fact in dignified 
language. He who believes in the struggle for existence and in the 
principle of natural selection, will acknowledge that eveiy organic 
being is constantly endeavouring to increase in numbers ; and that 
if any one being varies ever so little, either in habits or structure, 
and thus gains an advantage over some other inhabitant of the 
same country, it will seize on the place of that inhabitant, however 
different that may be from its own place. Hence it will cause him 
no surprise that there should be geese and frigate-birds with webbed 
feet, living on the dry land and rarely alighting on the water • 
that there should be long-toed corncrakes, living in meadows in- 
stead of in swamps ; that there should be woodpeckers where hardly 
a tree grows ; that there should be diving tlirushes and diving 
Hymenoptera, and petrels with the habits of auks. 

Organs of extreme Perfection and Complication. 

To suppose that the eye with all its inimitable contrivances for 
adjusting the focus to different distances, for admitting different 
amounts of light, and for the~c orrecti on of spherical and chromatic 
aberration, could have been formed by natiiraT" selection, seems, 
I freely confess, absurd in the highest degree. When it was first 
said that the sun stood still and the world turned round, the 
common sense of mankind declared the doctrine false ; but the old 
saying of Vox jpopulij vox Dei, as every philosopher knows, cannot 
be trusted in science. Reason tells me, that if numerous gradations 
from a simple and imperfect eye to one complex and perfect can 
bo shown to exist, each grade being; us eful to its possessor, as is 
certainly the case ; if further, the eye ever varies and the variations 
be inherited, as is likewise certainly the case ; and if such varia- 
tions should be useful to any animal under changing conditions of 
life, then the difficulty of believing that a perfect and complex ej^ 





could he formed by natural selection, though insuperable by our 
imaf^ination, should not be considered as subversive of the theory. 
How a nerve comes to be sensitive to light, hardly concerns us 
more than how life itself originated ; but I may remark that, as 
some of the lowest organisms, in which nerves cannot be Getected, 
are capable of perceiving light, it does not seem impossible that 
certain sensitive elements in their sarcode sh ould become aggrega ted 
and developed into nerves, endowed wit¥ this sp.eciad,§eji§ibility.'~ 

In searching for the gradations through which an organ in any 
species has been perfected, we ought to look exclusively to its lineal 
progenitors ; but this is scarcely ever possible, and we are forced to 
look to other species and genera of the same group, that is to the 
collateral descendants from the same parent-form, in order to see 
what gradations are possible, and for the chance of some gradations 
having been transmitted in an unaltered or little altered condition. 
But the state of the same organ in distinct classes may incidentally 
throw light on the steps by which it has been perfected. 

The simplest organ which can be called an eye consists of an 
optic nerve, surrounded by pigment-cells and covered by translucent 
skin, but without any lens or other refractive body. We may, 
however, according to M. Jourdain, descend even a step lower 
and find aggregates of pigment-cells, apparently serving as organs 
of vision, without any nerves, and resting merely on sarcodic tissue. 
Eyes of the above simple nature are not capable of distinct vision^ 
and serve only to distinguish light from darkness. In certain star- 
fishes, small depressions in the layer of pigment which surrounds 
the nerve are filled, as described by the author just quoted, with 
transparent gelatinous matter, projecting with a convex surface, like 
the cornea in the higher animals. He suggests that this serves not 
to form an image, but only to concentrate the luminous rays and 
render their perception more easy. In this concentration of the 
rays we gain the first and by far the most important step towards 
the formation of a true, picture-forming eye ; for we have only to 
place the naked extremity of the optic nerve, which in some of the 
lower animals lies deeply buried in the body, and in some near 
the surface, at the right distance from the concentrating apparatus, 
and an image will be formed on it. 

In the great class of the Articulata, we may start from an optic 
nerve simply coated with pigment, the latter sometimes forming a 
sort of pupil, but destitute of a lens or other optical contrivance. 
With insects it is now known that the numerous facets on the 
cornea of their great compound eyes form true lenses, and that 
the cones include curiously modified nervous filaments. But these 


organs in the Aiticulata are so much diversified that MUllcr fonnerly 
made three main classes with seven subdivisions, besides a fourth 
main class of aggregated simple eyes. 

When we reflect on these facts, here given much too briefly, with 
respect to th« wide, diversified, and graduated range of structure in 
the eyes of the lower animals ; and when we bear in mind how 
small the number of all living forms must be in comparison with 
those which have become extinct, the difficulty ceases to be very 
gi-eat in believing that natural selection may have converted the 
simple apparatus of an optic nerve, coated with pigment and 
invested by transparent membrane, into an optical instrument as 
perfect as is possessed by any member of the Articulate Class. 

He who will go thus far, ought not to hesitate to go one step 
fuither, if he finds on finishing this volume that large bodies 
of facts, otherwise inexplicable, can be explained by the theory of 
modification through natural selection ; he ought to admit that a 
structure even as perfect as an eagle's eye might thus be formed, 
although in this case he does not know the transitional states. It 
has been objected that in order to modify the eye and still preserve 
it as a perfect instrument, many changes would have to be effected 
simultaneously, which, it is assumed, could not be done through 
natural selection ; but as I have attempted to show in my work on 
the variation of domestic animals, it is not necessary to suppose 
that the modifications were all simultaneous, if they were extremely 
slight and gradual. Different kinds of modification would, also, 
serve for the same general purpose : as Mr. Wallace has remarked, 
" if a lens has too short or too long a focus, it may be amended 
either by an alteration of curvature, or an alteration of density ; if 
the curvature be irregular, and the rays do not converge to a point, 
then any increased regularity of curvature will be an improvement. 
So the contraction of the iris and the muscular movements of the 
eye are neither of them essential to vision, but only improvements 
which might have been added and perfected at any stage of the 
construction of the instrument." Within the highest division of 
the animal kingdom, namely, the Vertebrata, we can start from an 
eye so simple, that it consists, as in the lancelet, of a little sack of 
transparent skin, furnished with a nerve and lined with pigment, 
but destitute of any other apparatus. In fishes and reptiles, as 
Owen has remarked, " the range of gradations of dioptric structures 
is very great." It is a significant fact that even in man, according 
to the high authority of Virchow, the beautiful crystalline lens is 
formed in the embryo by an accumulation of epidermic cells, lying 
in a sack-like fold of the skin ; and the vitreous body is formed 



from embryonic sub-cutaneous tissue. To arrive, however, at a 
just conclusion regarding the formation of the eye, with all its mar- 
vellous yet not absolutely perfect characters, it is indispensable 
that the reason should conquer the imagination ; but I have felt 
the difficulty far too keenly to be surprised at others hesitating to 
extend the principle of natural selection to so startling a length. 

It is scarcely possible to avoid comparing the eye with a telescope. 
"We know that this instrument has been perfected by the long- 
continued efforts of the highest human intellects ; and we naturally 
infer that the eye has been formed by a somewhat analogous 
process. But may not this inference be presumptuous ? Have we 
any right to assume that the Creator works by intellectual povrers 
like those of man? If we must compare the eye to an optical 
instrument, we ought in imagination to take a thick layer of 
transparent tissue, with spaces filled with fluid, and with a nerve 
sensitive to light beneath, and then suppose every part of this layer 
to be continually changing slowly in density, so as to separate into 
layers of different densities and thicknesses, placed at different 
distances from each other, and with the surfaces of each la3''er 
clowly changing in form. Further we must suppose that there is a 
power, represented by natural selection or the survival of the fittest, 
always intently watching each slight alteration in the transparent 
layers ; and carefully preserving each which, under varied circum- 
stances, in any way or in any degree, tends to produce a distincter 
image. We must suppose each new state of the instrument to be 
/nultii)lied by the million ; each to be preserved until a better one 
is produced, and then the old ones to be all destroyed. In living 
bodies, variation will cause the slight alterations, generation will 
multiply them almost infinitely, and natural selection will pick out 
with unerring skill each improvement. Let this process go on for 
millions of years ; and during each year on millions of individuals 
of many kinds ; and may we not believe that a living optical 
instrument might thus be formed as sujjerior to one of glass, as the 
works of the Creator are to those of man ? 

Modes of Transition. 

If it could be demonstrated that any complex organ existed, 
which could not possibly have been formed by numerous, suc- 
cessive, slight modifications, my theory would absolutely break 
down. But I can find out no such case. No doubt many organs 
exist of which we do not know the transitional grades, more espe- 
cially if we look to m-uch-isolated species, round which, according to 
the theory, there has been mich extinction. Or again, if we take 


an organ common to all the members of a class, for in this latter 
case the organ must have been originally formed at a remote period, 
since which all the many members of the class have been developed; 
and in order to discover the early transitional grades through which 
the organ has passed, we should have to look to very ancient ances- 
tral forms, long since become extinct. 

We should be extremely cautious in concluding that an organ 
could not have been formed by transitional gradations of some kind. 
Numerous cases could be given amongst the lower animals of the 
same organ performing at the same time wholly distinct functions ; 
thus in the larva of the dragon-fly and in the fish Cobites the 
alimentary canal respires, digests, and excretes. In the Hydra, the 
animal may be turned inside out, and the exterior surface will then 
digest and the stomach respire. In such cases natural selection 
might specialise, if any advantage were thus gained, the whole or 
part of an organ, which had previously performed two functions, for 
one function alone, and thus by insensible steps greatly change its 
nature. Many plants are known which regularly produce at the 
same time differently constructed flowers ; and if such plants were 
to produce one kind alone, a great change would be effected with 
comparative suddenness in the character of the species. It is, how- 
ever, probable that the two sorts of flowers borne by the same plant 
were originally differentiated by finely graduated steps, which may 
still be followed in some few cases. 

Again, two distinct organs, or the same organ under two very 
different forms, may simultaneously perform in the same individual 
the same function, and this is an extremely important means of 
transition : to give one instance, — there are fish with gills or 
branchia3 that breathe the air dissolved in the water, at the same 
time that they breathe free air in their swimbladders, this latter 
organ being divided by highly vascular partitions, and having a 
ductus pneumaticus for the supply of air. To give another instance 
from the vegetable kingdom : plants climb by three distinct means, 
by spirally twining, by clasping a support with their sensitive 
tendrils, and by the emission of aerial rootlets ; these three means 
are usually found in distinct groups, but some few species exhibit 
two of the means, or even all three, combined in the same indivi- 
dual. In all such cases one of the two organs might readily be 
modified and perfected so as to perform all the work, being aided 
during the progress of modification by the other organ ; and then 
this other organ might be modified for some other and quite distinct 
purpose, or be wholly obliterated. 
^ The illustration of the swimbladder in fishes is a good one. 


because it shows us clearly the highly important fact that an organ 
originally constructed for one purpose, namely, flotation, may be 
converted into one for a widely different purpose, namely, respiration. 
The swimbladder has, also, been worked in as an accessory to the 
auditory organs of certain fishes. All physiologists admit that 
the swimbladder is homologous, or " ideally similar " in position and 
structure with the lungs of the higher vertebrate animals : hence 
there is no reason to doubt that the swimbladder has actually 
been converted into lungs, or an organ used exclusively for 

According to this view it may be inferred that all vertebrate 
animals with true lungs are descended by ordinary generation from 
an ancient and unknown prototype, which was furnished with a 
floating apparatus or swimbladder. We can thus, as I infer from 
Owen's interesting description of these parts, understand the strange 
fact that every particle of food and drink which we swallow has to 
pass over the orifice of the trachea, with some risk of falling into 
the lungs, notwithstanding the beautiful contrivance by which the 
glottis is closed. In the higher Vertebrata the branchice have 
wholly disappeared — but in the embryo the slits on the sides of the 
neck and the loop-like course of the arteries still mark their former 
position. But it is conceivable that the now utterly lost branchire 
might have been gradually worked in by natural selection for some 
distinct purpose : for instance, Landois has shown that the wings 
of insects are developed from the tracheee ; it is therefore highly 
probable that in this great class organs which once served for 
respiration have been actually converted into organs for flight. 

In considering transitions of organs, it is so important to bear in 
mind the probability of conversion from one function to another, 
that I will give another instance. Pedunculated cirripedes have 
two minute folds of skin, called by me the ovigerous frena, which 
fierve, through the means of a sticky secretion, to retain the eggs 
until they are hatched within the sack. These cirripedes have no 
branchise, the whole surface of the body and of the sack, together 
with the small frena, serving for respiration. The Balanidas or 
sessile cirripedes, on the other hand, have no ovigerous frena, the 
eggs lying loose at the bottom of the sack, within the well-enclosed, 
^shell ; but they have, in the same relative position with the frena, 
large, much-folded membranes, which freely communicate with the 
circulatory lacunas of the sack and body, and which have been 
considered by all naturalists to act as branchiae. Now I think no 
one will dispute that the ovigerous frena in the one family are) 
strictly homologous with the brancL'^e of the other family ; indeed,! 


they graduate into each other. Therefore it need not be doubted 
that the two little folds of skin, which originally served as 
ovigerous frena, but which, likewise, very slightly aided in the 
act of respiration, have been gradually converted by natural 
selection into branchiae, simi)ly through an increase in their size 
and the obliteration of their adhesive glands. If all pedunculated 
cirripedes had become extinct, and they have suffered far more 
extinction than have sessile cirripedes, who would ever have 
imagined that the branchiae in this latter family had originally 
existed as organs for preventing the ova from being washed out of 
the sack ? 

There is another possible mode of transition, namely, through 
the acceleration or retardation of the period of reproduction. This 
has lately been insisted on by Prof. Cope and others in the United 
States. It is now known that some animals are capable of repro- 
duction at a very early age, before they have acquired their perfect 
characters ; and if this power became thoroughly well developed in 
a species, it seems probable that the adult stage of development 
would sooner or later be lost ; and in this case, especially if the 
larva differed much from the mature form, the character of the 
species would be greatly changed and degraded. Again, not a few 
animals, after arriving at maturity, go on changing in character 
during nearly their whole lives. With mammals, for instance, the 
form of the skull is often much altered with age, of which Dr. 
Murie has given some striking instances with seals ; every one 
knows how the horns of stags become more and more branched, 
and the plumes of some birds become more finely developed, as 
they grow older. Prof. Cope states that the teeth of certain lizards 
change much in shape with advancing years ; with crustaceans not 
only many trivial, but some important parts assume a new 
character, as recorded by Fritz Miiller, after maturity. In all such 
cases, — and many could be given, — if the age for reproduction were 
retarded, the character of the species, at least in its adult state, 
would be modified; nor is it improbable that the previous and 
earlier stages of development would in some cases be hurried 
through and finally lost. Whether species have often or ever been 
modified through this comparatively sudden mode of transition, 1 
can form no opinion ; but if this has occurred, it is probable that 
the differences between the young and the mature, and betweeo 
the mature and the old, were primordially acquired by graduated 


SpecLil Difficulties of the Theory of Natural Selection. 

Although we must be extremely cautious in concluding that any 
organ could not have been produced by successive, small, tran- 
sitional gradations, yet undoubtedly serious cases of difficulty occur. 

One of the most serious is that of neuter insects, which are often 
differently constructed from either the males or fertile females ; but 
this case will be treated of in the next chapter. The electric 
organs of fishes offer another case of special difficulty; for it is 
impossible to conceive by what steps these wondrous organs have 
been produced. But this is not surprising, for we do not even 
know of what use they are. In the Gymnotus aud Torpedo they 
no doubt serve as powerful means of defence, and perhaps fot 
securing prey; yet in the Eay, as observed by Matteucci, an 
analogous organ in the tail manifests but little electricity, even 
when the animal is greatly irritated; so little, that it can 
hardly be of any use for the above purposes. ]\Torcover, in the 
Hay, besides the organ just referred to, there is, as Dr. R. M'Donneil 
has shown, another organ near the head, not known to be electrical, 
but which appears to be the real homologue of the electric battery 
in the Torpedo. It is generally admitted that there exists between 
these organs and ordinary muscle a close analogy, in intimate 
structure, in the distribution of the nerves, and in the manner in 
which they are acted on by various reagents. It should, also, be 
especially observed that muscular contraction is accompanied by an 
electrical discharge ; and, as Dr. Eadcliffe insists, " in the electrical 
apparatus of the torpedo during rest, there would seem to be a 
charge in every respect like that which is met with in muscle and 
nerve during rest, and the discharge of the torpedo, instead of being 
peculiar, may be only another form of the discharge which attends 
upon the action of muscle and motor nerve." Beyond this we can- 
not at present go in the way of explanation ; but as we know so 
little about the uses of these organs, and as we know nothing about 
the habits and structure of the progenitors of the existing electric 
fishes, it w^ould be extremely bold to maintain that no serviceable 
transitions are possible by which these organs might have been 
gradually developed. 

These organs appear at first to offer another and far more serious 
difficulty ; for they occur in about a dozen kinds of fish, of which 
several are widely remote in their affinities. When the same organ 
is fouDd in several members of the same class, especially if in 
mcmbeis having very different habits of life, we may generally 
attribute its presence to inheritance from a common ancestor ; a:id 


its absence in some of the members to loss through disuse or 
natural selection. So that, if the electric organs had been 
inherited from some one ancient progenitor, we might have 
expected that all electric fishes would have been specially related 
to each other ; but this is far from the case. Nor does geology at 
all lead to the belief that most fishes formerly possessed electric 
organs, which their modified descendants have now lost. But 
when we look at the subject more closely, we find in the several 
fishes provided with electric organs, that these are situated in dif- 
ferent parts of the body, — that they differ in construction, as in 
the arrangement of the plates, and, according to Pacini, in the 
process or means by which the electricity is excited — and lastly, in 
being supplied with nerves proceeding from different sources, and 
this is perhaps the most important of all the differences. Hence in 
the several fishes fi:./nished with electric organs, these cannot be 
considered as homologous, but only as analogous in function. Con- 
sequently there is no reason to suppose that they have been inherited 
from a common progenitor ; for had this been the case they would 
have closely resembled each other in all respects. Thus the difficulty 
of an organ, apparently the same, arising in several remotely allied 
species, disappears, leaving only the lesser yet still great difiSculty ; 
namely, by what graduated steps these organs have been developed 
in each separate group of fishes. 

The luminous organs which occur in a few insects, belonging 
to widely different families, and which are situated in different 
parts of the body, offer, under our present state of ignorance, a 
difficulty almost exactly parallel with that of the electric organs. 
Other similar cases could be given ; for instance in plants, the very 
curious contrivance of a mass of pollen-grains, borne on a foot-stalk 
with an adhesive gland, is apparently the same in Orchis and 
Asclepias, — genera almost as remote as is possible amongst flowering 
plants ; but here again the parts are not homologous. In all cases 
of beings, far removed from each other in the scale of organisation, 
which are furnished with similar and peculiar organs, it will be 
found that although the general appearance and function of the 
organs may be the same, yet fundamental differences between them 
can always be detected. For instance, the eyes of cephalopods 
or cuttle-fish and of vertebrate animals appear wonderfully alike ; 
and in such widely sundered groups no part of this resemblance can 
be due to inheritance from a common progenitor. Mr. Mivart has 
advanced this case as one of special difficulty, but I am unable to 
see the force of his argument. An organ for vision must be formed 
oi transparent tissue, and must include some sort cf lens fot 


throwing an image at the back of a darkened chamber. Beyond 
this superficial resemblance, there is hardly any real similarity 
between the eyes of cuttle-fish and vertebrates, as may be seen by 
cx)nsulting Hensen's admirable memoir on these organs in the 
Cephalopoda. It is impossible for me here to enter on details, but 
I may specify a few of the points of difference. The crystalline lens 
in the higher cuttle-fish consists of two parts, placed one behind the 
other like two lenses, both having a very different structure and 
disposition to what occurs in the vertebrata. The retina is wholly 
different, with an actual inversion of the elemental parts, and with 
a large nervous ganglion included within the membranes of the 
eye. The relations of the muscles are as different as it is pos- 
sible to conceive, and so in other points. Hence it is not a little 
difficult to decide how far even the same terms ought to be employed 
in describing the eyes of the Cephalopoda and Vertebrata. It is, of 
course, open to any one to deny that the eye in either case could 
liave been developed through the natural selection of successive, 
slight variations ; but if this be admitted in the one case, it is 
clearly possible in the other ; and fundamental differences of struc- 
ture in the visual organs of two groups might have been anti- 
cipated, in accordance with this view of their manner of formation. 
As two men have sometimes independently hit on the same 
invention, so in the several foregoing cases it appears that natural 
selection, working for the good of each t:>emg, and taking advan- 
tage of all favourable variations, has produced similar organs, as 
far as function is concerned, in distmct organic beings, which 
o^ve none of their structure in common to inheritance from a 
common progenitor. 

Fritz MtiUer, in order to test the conclusions arrived at in this 
volume, has followed out with much care a nearly similar line of 
argument. Several families of crustaceans include a few species, 
possessing an air-breathing apparatus and fitted to live out of the 
water. In two of these families, which were more especially 
examined by MUller, and which are nearly related to each other, 
the species agree most closely in all important characters ; namely 
in their sense-organs, circulating system, in the porition of the 
tufta of hair within their complex stomachs, and [lastly in the 
whole structure of the water-breathing branchiae, even to the 
microscopical hooks by which they are cleansed. Hence it might 
have been expected that in the few species belonging to both 
families which live on the land, the equally-important air-breathing 
apparatus would have been the same ; for why should this one 
apparatus, g-ven for the same purpose, have been made to differ, 


whilst all the other important organs were closely similar or rather 

Fritz Miiller argues that this close similarity in so many points 
of structure must, in accordance with the views advanced by me» 
be accounted for by inhoitance from a common progenitor. Bui 
as the vast majority of the species in the above two families, as 
well as most other crustaceans, are aquatic in their habits, it is 
improbable in the highest degree, that their common progenitor 
should have been adapted for breathing air. Miiller was thus led 
carefully to examine the apparatus in the air-breathing species; 
and he found it to differ in each in several important points, as in 
the position of the orifices, in the manner in which they are opened 
and closed, and in some accessory details. Now such differences 
are intelligible, and might even have been expected, on the suppo- 
sition that species belonging to distinct families had slowly become 
adapted to live more and more out of water, and to breathe the 
air. For these species, from belonging to distinct families, would 
have differed to a certain extent, and in accordance with the 
principle that the nature of each variation depends on two factors, 
viz. the nature of the organism and that of the surrounding con- 
ditions, their variability assuredly would not have been exactly the 
same. Consequently natural selection would have had different 
materials or variations to work on, in order to arrive at the same 
functional result ; and the structures thus acquired would almost 
necessarily have differed. On the hypothesis of separate acts of 
creation the whole case remains unintelligible. This line of 
argument seems to have had great weight in leading Fritz Miiller 
to accept the views maintained by me in this volume. 

Another distinguished zoologist, the late Professor Claparede, has 
argued in the same manner, and has arrived at the same result. 
He shows that there are parasitic mites (Acaridas), belonging to 
distinct sub- families and families, which are furnished with hair- 
claspers. These organs must have been independently developed, 
as they could not have been inherited from a common progenitor ; 
and in the several groups they are formed by the modification of 
the fore-legs, — of the iiind-legs, — of the maxillre or lips, — and of 
appendages on the under side of the hind part of the body. 

In the foregoing cases, we see the same end gained and the same 
function performed, in beings not at all or only remotely allied, by 
organs in appearance, though not in development, closely similar. 
On the other hand, it is a common rule throughout nature that the 
Rhine end should be gained, even sometimes in the case of closely- 
reiatetl beings, by the most diversified means. How differertly 


constructed is the feathered wing of a bird and the membrane- 
covered wing of a bat ; and still more so the four wings of a butter- 
fly, the two wings of a fly, and the two wings with the elytra of a 
beetle. Bi valve shells are made to open and shut, but on what a 
number of patterns is the hinge constructed, — from the long row of 
neatly interlocking teeth in a Nucula to the simple ligament of a 
Mussel ! Seeds are disseminated by their minuteness,— by their 
capsule being converted into a light balloon-like envelope, — by 
being embedded in pulp or flesh, formed of the most diverse parts, 
and rendered nutritious, as well as conspicuously coloured, so as to 
attract and be devoured by birds, — by having hooks and grapnels 
of many kinds and serrated awns, so as to adhere to the fur of 
quadrupeds, — and by being furnished with wings and plumes, as 
different in shape as they are elegant in structure, so as to be wafted 
by every breeze. I will give one other instance ; for this subject of 
the same end being gained by the most diversified means well 
deserves attention. Some authors maintain that organic beings 
have been formed in many ways for the sake of mere variety, 
almost like toys in a shop, but such a view of nature is incredible. 
With plants having separated sexes, and with those in which, 
though hermaphrodites, the pollen does not spontaneously fall on 
the stigma, some aid is necessary for their fertilisation. With 
several kinds this is effected by the pollen-grains, which are light 
and incoherent, being blown by the wind through mere chance on 
to the stigma ; and this is the simplest plan which can well bo 
conceived. An almost equally simple, though very different, plan 
occurs in many plants in which a symmetrical flower secretes a fevj 
deops of nectar, and is consequently visited by insects ; and these 
c^rry the pollen from the anthers to the stigma. 

From this simple stage we may pass through an inexhaustible 
number of contrivances, all for the same purpose and effected in 
essentially the same manner, but entailing changes in every part of 
the flower. The nectar may be stored in variously shaped recep- 
tacles, with the stamens and pistils modified in many ways, some- 
times forming trap-like contrivances, and sometimes capable of 
neatly adapted movements through irritability or elasticity. From 
such structures we may advance till we come to such a case of 
extraordinary adaptation as that lately described by Dr. Criiger in 
the Coryanthes. This orchid has part of its labellum or lower lip 
hoUowed out into a great bucket, into which drops of almost pure 
water continually fall from two secreting horns which stand above 
it ; and when the bucket is half full, the water overflows by a 
spout on one side. The basal part of the labellum stands over the 
bucket, and is itself hollowed out into a sort of chamber with two 


lateral entrances ; within this chamber there are curious fleshy 
ridges. The most ingenious man, if he had not witnessed what 
takes place, could never have imagined what purpose all these parts 
serve. But Dr. Criiger saw crowds of large humble-bees visiting 
the gigantic flowers of this orchid, not in order to suck nectar, but 
to arnaw off the ridges within the chamber above the bucket ; in 
doing this they frequently pushed each other into the bucket, and 
their wings being thus wetted they could not fly away, but were 
compelled to crawl out through the passage formed by the spout or 
overflow. Dr. Criiger saw a " continual procession " of bees thus 
crawling out of their involuntary bath. The passage is narrow, and 
is roofed over by the column, so that a bee, in forcing its way out, 
first rubs its back against the viscid stigma and then against the 
viscid glands of the pollen -masses. The pollen-masses are thus 
glued to the back of the bee which first happens to crawl out 
through the passage of a lately expanded flower, and are thus 
carried away. Dr. Criiger sent me a flower in spirits of wine, with 
a bee which he had killed before it had quite crawled out with ?» 
poUen-inass still fastened to its back. When the bee, thus provided, 
flies to another flower, or to the same flower a second time, and is 
pushed by its comrades into the bucket and then crawls out by the 
passage, the pollen-mass necessarily comes first into contact with 
I he viscid stigma, and adheres to it, and the flower is fertilised. 
Now at last we see the full use of every part of the flower, of the 
water-secreting horns, of the bucket half full of water, which 
prevents the bees from flying away, and forces them to crawl out 
through the spout, and rub against the properly placed viscid pollen- 
masses and the viscid stigma. 

The construction of the flower in another closely allied orchid, 
namely the Catasetum, is widely different, though serving the same 
end ; and is equally curious. Bees visit these flowers, like those of 
the Coryanthes, in order to gnaw the labellum ; in doing this they 
inevitably touch a long, tapering, sensitive projection, or, as I have 
called it, the antenna. This antenna, when touched, transmits a 
sensation or vibration to a certain membrane which is instantly 
ruptured ; this sets free a spring by which the pollen-mass is shot 
forth, like an arrow, in the right direction, and adheres by its 
viscid extremity to the back of the bee. The pollen-mass of the 
male plant (for the sexes are separate in this orchid) is thus carried 
to the flower of the female plant, where it is brought into contact 
with the stigma, which is viscid enough to break certain elastic 
threads, and retaining the pollen, fertilisation is effected. 

How, it may be asked, in the foregoing and in ninumerable other 


instances, can we understand the graduated scale of complexity ajid 
the multifarious means for gaining the same end. The answer no 
doubt is, as already remarked, that when two forms vary, which 
abeady differ from each other in some slight degree, the variability 
will not be of the same exact nature, and consequently the results 
obtained through natural selection for the same general purpose will 
not be the same. We should also bear in mind that every highly 
developed organism has passed through many changes; and that 
each modified structure tends to be inherited, so that each modi- 
fication will not readily be quite lost, but may be again aud again 
further altered. Hence the structure of each part of each species, 
for whatever purpose it may serve, is the sum of many inherited 
changes, through which the species has passed during its successive 
adaptations to changed habits and conditions of life. 

Finally then, although in many cases it is most difficult even to 
conjecture by what transitions organs have arrived at their present 
state ; yet, considering how small the proportion of living and known 
forms is to the extinct and unknown, I have been astonished how 
rarely an organ can be named, towards which no transitional grade 
is known to lead. It certainly is true, that new organs appearing 
as if created for some special purpose, rarely or never appear in any 
being ; — as indeed is shown by that old, but somew^hat exaggerated, 
canon in natural history of " Natura non facit saltum." We meet 
with this admission in the writings of almost every experienced 
naturalist; or as Milne Edwards has well expressed it, Nature is 
prodigal in variety, but niggard in innovation. Why, on the 
theory of Creation, should there be so much variety and so little 
real novelty? Why should all the parts and organs of many 
independent beings, each supposed to have been separately created 
for its proper place in nature, be so commonly linked together by 
graduated steps ? Why should not Nature take a sudden leap 
from structure to structure? On the theory of natural selection, 
"we can clearly understand why she should not ; for natural 
selection acts only by taking advantage of slight successive varia- 
tions ; she can never take a great and sudden leap, but must 
advance by short and sure, though slow steps. 

Organs of little apparent Importance, as affected hy Natural 


As natural selection acts by life and death, — by the survival oi 
the fittest, and by the destruction of the less well-fitted indi- 
viduals, — 1 have sometimes felt great difficulty in understanding 
the origin or formation of parts of little importance; almost as 


great, though of a very different kind, as in the case of the most 
perfect and complex organs. 

In the first place, we are much too ignorant in regard to the 
whole economy of any one organic being, to say what slight modifi- 
cations would be of importance or not. In a former chapter I have 
given instances of very trifling characters, such as the clown on 
fruit and the colour of its flesh, the colour of the skin and hair of 
quadrupeds, which, from being correlated with constitutional 
differences or from determining the attacks of insects, might 
assuredly be acted on by natural selection. The tail of the giraffe 
looks like an artificially constructed fly-flapper ; and it seems at 
first incredible that this could have been adapted for its present 
purpose by successive slight modifications, each better and better 
fitted, for so trifling an object as to drive away flies ; yet we should 
pause before being too positive even in this case, for we know that 
the distribution and existence of cattle and other animals in South 
America absolutely depend on their power of resisting the attacks 
of insects : so that individuals which could by any means defend 
themselves from these small enemies, would be able to range into 
new pastures and thus gain a great advantage. It is not that the 
larger quadrupeds are actually destroyed (except in some rare 
cases) by flies, but they are incessantly harassed and their strength 
reduced, so that they are more subject to disease, or not so well 
enabled in a coming dearth to search for food, or to escape from 
beasts of prey. 

Organs now of trifling importance have probably in some cases 
been of high importance to an early progenitor, and, after having 
been slowly perfected at a former period, have been transmitted to 
existing species in nearly the same state, although now of very 
slight use ; but any actually injurious deviations in their structure 
would of course have been checked by natural selection. Seeing 
how important an organ of locomotion the tail is in most aquatic 
animals, its general presence and use for many purposes in so 
many land animals, v/hich in their lungs or modified swimbladders 
betray their aquatic origin, may perhaps be thus accounted for. A 
well-developed tail having been formed in an aquatic animal, it 
might subsequently come to be worked in for all sorts of purposes, 
— as a fly-flapper, an organ of prehension, or as an aid in turning, as 
in the case of the dog, though the aid in this latter respect must be 
slight, for the hare, with hardly any tail, can double still more 

In the second place, we may easily err in attributing importance 
to characters, and in believing that they have been developed 


through natural selection. We must by no means overlook the 
effects of the definite action of changed conditions of life, — of so- 
called spontaneous variations, which seem to depend in a quite 
subordinate degree on the nature of the conditions, — of the ten- 
dency to reversion to long-lost characters,— of the complex laws of 
growth, such as of correlation, compensation, of the pressure of one 
part on another, &c.,— and finally of sexual selection, by which 
characters of use to one sex are often gained and then transmitted 
more or less perfectly to the other sex, though of no use to this 
sex. But structures thus indirectly gained, although at first of no 
advantage to a species, may subsequently have b(;en taken advan- 
tage of by its modified descendants, under new conditions of life 
and newly acquired habits. 

If green woodpeckers alone had existed, and we did not know 
that there were many black and pied kinds, I dare say that we 
should have thought that the green colour was a beautiful adapta- 
tion to conceal this tree-frequenting bird from its enemies ; and 
consequently that it was a character of importance, and had been 
acquired through natural selection ; as it is, the colour is probably 
in chief part due to sexual selection. A trailing palm in the Malay 
Archipelago climbs the loftiest trees by the aid of exquisitely con- 
structed hooks clustered around the ends of the branches, and this 
contrivance, no doubt, is of the highest service to the plant ; but as we 
see nearly similar hooks on many trees which are not climbers, and 
which, as there is reason to believe from the distribution of the thorn- 
bearing species in Africa and South America, serve as a defence 
against browsing quadrupeds, so the spikes on the palm may at 
first have been developed for this object, and subsequently have been 
improved and taken advantage of by the plant, as it underwent 
further modification and became a climber. The naked skin on the 
head of a vulture is generally considered as a direct adaptation for 
wallowing in putridity ; and so it may be, or it may possibly be 
due to the direct action 3f putrid matter ; but we should be very 
cautious in drawing any such inference, when we see that the skin 
on the head of the clean-feeding male Turkey is likewise naked. 
The sutures in the skulls of young mammals have been advanced aa 
a beautiful adaptation for aiding parturition, and no doubt they 
facilitate, or may be indispensable for this act ; but as sutures 
occur in the skulls of young birds and reptiles, which have only to 
escape from a broken egg, we may infer that this structure has 
arisen from the laws of growth, and has been taken advantage of ir 
the parturition of the higher animals. 

We are profoundly ignorant of the cause of each slight varia' ion 


or individual difterence ; and we are immediately made conscions 
ci" this by reflecting on the difTerences between the breeds of our 
domesticated animals in different countries, — more especially in the 
less civilised countries where there has been but little methodical 
selection. Animals kept by savages in difterent countries often 
Lave to struggle for their own subsistence, and are exposed to a 
certain extent to natural selection, and individuals with slightly 
dillerent constitutions would succeed best under different climates. 
With cattle susceptibility to the attacks of flies is correlated with 
colour, as is the liability to be poisoned by certain plants ; so that 
even colour would be thus subjected to the action of natural 
selection. Some observers are convinced that a damp climate 
affects the growth of the hair, and that with the hair the horns are 
correlated Mountain breeds always differ from lowland breeds ; 
and a mountainous country would probably affect the hind limbs 
from exercising them more, and possibly even the form of the 
pelvis ; and then by the law of homologous variation, the front 
limbs and the head would probably be affected. The shape, also, 
of the pelvis might affect by pressure the shape of certain parts of 
the young in the w^omb. The laborious breathing necessary in 
high regions tends, as we have good reason to believe, to increase 
tl^.e size of the chest ; and again correlation would come into play. 
The effects of lessened exercise together with abundant food on the 
whole organisation is probably still more important ; and this, as 
H. von Nathusius has lately shown in his excellent Treatise, is 
apparently one chief cause of the great modification which the 
breeds of swine have undergone. But we are far too ignorant to specu- 
late on the relative importance of the several known and unknown 
causes of variation ; and I have made these remarks only to show 
that, if we are unable to account for the characteristic differences 
of our several domestic breeds, which nevertheless are generally 
admitted to have arisen through ordinary generation from one or a 
few parent-stocks, we ought not to lay too much stress on our 
ignorance of the precise cause of the slight analogous differences 
between true species. 

Utilitarian Doctrine^ Tiowfar true : Beauty, liow acquired. 

The foregoing remarks lead me to say a few words on the protest 
lately made by some naturalists, against the utilitarian doctrine 
that every detail of structure has been produced for the good of its 
[assessor. They believe that many structures have been created 
for the sake of beauty, to delight man or the Creator (but this 
latter point is beyond ths scope of scientific discussion), or for the 


sake of mere variety, a view already discussed. Such doctrines, if 
true, would be absolutely fatal to my theory. I fully admit that 
many structures are now of no direct use to their possessors, and 
may never have been of any use to their progenitors ; but this does 
not prove that they were formed solely for beauty or variety. Nc 
doubt the definite action of changed conditions, and the various 
causes of modifications, lately specified, have all produced an 
effect, probably a great effect, independently of any advantage thus 
gained. But a still more important consideration is that the chiel 
part of the organisation of every living creature is due to inherit- 
ance ; and consequently, though each being assuredly is well fitted 
for its place in nature, many structures have now no very close and 
direct relation to present habits of life. Thus, we can hardly 
believe that the webbed feet of the upland goose or of the frigate- 
bird are of special use to these birds ; we cannot believe that the 
similar bones in the arm of the monkey, in the fore-leg of the 
horse, in the wing of the bat, and in the flipper of the seal, are of 
special use to these animals. We may safely attribute these 
structures to inheritance. But webbed feet no doubt were as 
useful to the progenitor of the upland goose and of the frigate-bird, 
as they now are to the most aquatic of living birds. So we may 
believe that the progenitor of the seal did not possess a flipper, but 
a foot with five toes fitted for walking or grasping ; and we may 
further venture to believe that the several bones in the limbs of 
the monkey, horse, and bat, were originally developed, on the 
principle of utility, probably through the reduction of more 
numerous bones in the fin of some ancient fish-like progenitor of 
the whole class. It is scarcely possible to decide how much 
allowance ought to be made for such causes of change, as the 
definite action of external conditions, so-called spontaneous varia- 
tions, and the complex laws of growth ; but v/ith these important 
exceptions, we may conclude that the structure of every living 
creature either now is, or was formerly, of some direct or in- 
direct use to its possessor. 

With respect to the belief that organic beings have been created 
beautiful for the delight of man, — a belief which it has been pro- 
nounced is subversive of my whole theory, — I may first remark 
that the sense of beauty obviously depends on the nature of the 
mind, irrespective of any real quality in the admired object ; and 
that the idea of what is beautiful, is not innate or unalterable. 
We see this, for instance, in the men of different races admiring an 
entirely different standard of beauty in their women. If beautiful 
objects had been created solely for man's gratification, it ought to 


be shown that before man appeared, there was less beauty on the 
face of the earth than since he came on the stage. Were the beau- 
tiful volute and cone shells of the Eocene epoch, and the gracefully 
Bculptui'ed ammonites of the Secondary period, created that man 
might ages afterwards admire them in his cabinet ? Few objects 
are more beautiful than the minute siliceous cases of the diato- 
macese: were these created that they might be examined and 
admired under the higher powers of the microscope ? The beauty 
in this latter case, and in many others, is apparently wholly due 
to symmetry of growth. Flowers rank amongst the most beautiful 
productions of nature ; but they have been rendered conspicuous 
in contrast with the green leaves, and in consequence at the same 
time beautiful, so that they may be easily observed by insects. I 
have come to this conclusion from finding it an invariable rule that 
when a flower is fertihsed by the wind it never has a gaily-coloured 
corolla. Several plants habitually produce two kinds of flowers ; 
one kind open and coloured so as to attract insects; the other 
closed, not coloured, destitute of nectar, and never visited by insects. 
Hence we may conclude that, if insects had not been developed on the 
face of the earth, our plants would not have been decked with beau- 
tiful flowers, but would have produced only such poor flowers as we 
see on our flr, oak, nut and ash trees, on grasses, spinach, docks, and 
nettles, which are all fertilised through the agency of the wind. A 
similar line of argument holds good with fruits ; that a ripe straw- 
berry or cherry is as pleasing to the eye as to the palate, — that the 
gaily-coloured fruit of the spindle-wood tree and the scarlet berries 
of the holly are beautiful objects, — will be admitted by every one,, 
But this beauty serves merely as a guide to birds and beasts, in 
order that the fruit may be devoured and the manured seeds dis- 
seminated : I infer that this is the case from having as yet found 
no exception to the rule that seeds are always thus disseminated 
when embedded within a fruit of any kind (that is within a fleshy 
or pulpy envelope), if it be coloure<l of any brilliant tint, or ren- 
dered conspicuous by being white or black. 

On the other hand, I willingly admit that a great number of 
male animals, as all our most gorgeous birds, some fishes, reptiles, 
and mammals, and a host of magnificently coloured butterflies, 
have been rendered beautiful for beauty's sake ; but this has been 
eifected through sexual selection, that is, by the more beautiful 
males having been continually preferred by the females, and^not for 
the delight of man. So it is with the music of birds. We may 
inff from all this that a nearly similar taste for beautiful colours 
and for musical sounds runs through a large part of the animal 



kingdom. When the female is as beautifully coloured as the male, 
which is not rarely the case with birds and butterflies, the cause 
apparently lies in the colours acquired through sexual selection 
having been transmitted to both sexes, instead of to the males 
alone. How the sense of beauty in its simplest form — that is, the 
reception of a peculiar kind of pleasure from certain colours, forms , 
and sounds — was first developed in the mind of man and of the lower 
animals, is a very obscure subject. The same sort of difficulty is 
presented, if we enquire how it is that certain flavours and odours 
give pleasure, and others displeasure. Habit, in all these cases 
appears to have come to a certain extent into play ; but there must 
be some fundamental cause in the constitution of the nervous 
system in each species. 

Natural selection cannot possibly produce any modification in 
a species exclusively for the good of another species ; though 
throughout nature one species incessantly takes advantage of, and 
profits by, the structures of others. But natural selection can 
and does often produce structures for the direct injury of other 
animals, as we see in the fang of the adder, and in the ovipositor 
of the ichneumon, by which its eggs are deposited in the living 
bodies of other insects. If it could be proved that any part of 
the structure of any one species had been formed for the exclusive 
good of another species, it would annihilate my theory, for such 
could not have been produced through natural selection. Although 
many statements may be found in works on natural history to this 
effect, I cannot find even one which seems to me of any weight. 
It is admitted that the rattlesnake has a poison-fang for its own 
defence, and for the destruction of its prey; but some authors 
suppose that at the same time it is furnished with a rattle for its own 
injury, namely, to warn its prey. I would almost as soon believe 
that the cat curls the end of its tail when preparing to spring, in 
order to warn the doomed mouse. It is a much more probable 
view that the rattlesnake uses its rattle, the cobra expands its frilly 
and the puff-adder swells whilst hissing so loudly and harshly, in 
order to alarm the many birds and beasts which arc known to 
attack even the most venomous species. Snak es act on the same 
principle which makes the hen ruffle her feathers and expand her 
wings when a dog approaches her chickens ; but I have not space 
hsreto enlarge on the many ways by which animals endeavour to 
frighten away their enemies. 

Natural selection will never produce in a being any structure 
more injurious than, beneficial to that being, for natural selection 


acts solely by and for the good of each. No organ will be formed 
as Paley has remarked, for the purpose of causing pain or for doinf» 
an injury to its possessor. If a fair balance be struck between the 
good and evil caused by each part, each will be found on the whole 
advantageous. After the lapse of time, under changing conditions 
of life, if any part comes to be injurious, it will be modified ; or if it 
be not so, the being will become extinct as myriads have become 

Natural selection tends only to make each organic being as 
perfect as, or slightly more perfect than, the other inhabitants of the 
same country with which it comes into competition. And we see 
that this is the standard of perfection attained under natm'e. The 
endemic productions of New Zealand, for instance, are perfect one 
compared with another ; but they are now rapidly yielding before 
the advancing legions of plants and animals introduced from Europe. 
Natural selection will not produce absolute perfection, nor do we 
always meet, as far as we can judge, with this high standard under 
nature. The correction for the aberration of light is said by Miiller 
not to be perfect even in that most perfect organ, the human eye. 
Helmholtz, whose judgment no one will dispute, after describing 
in the strongest terms the wonderful powers of the human eye, 
adds these remarkable words : " That which we have discovered 
in the way of inexactness and imperfection in the optical machine 
and in the image on the retina, is as nothing in comparison with the 
incongruities which we have just come across in the domain of the 
sensations. One might say that nature has taken delight in accu- 
mulating contradictions in order to remove all foundation from the 
theory of a pre-existing harmony between the external and internal 
worlds." If our reason leads us to admire with enthusiasm a 
multitude of inimitable contrivances in nature, this same reason 
tells us, though we may easily err on both sides, that some other 
contrivances are less perfect. Can we consider the sting of the bee 
as perfect, which, when used against many kinds of enemies, 
cannot be withdrawn, owing to the backward serratures, and thus 
inevitably causes the death of the insect by tearing out its viscera ? 

If we look at the sting of the bee, as having existed in a remote 
progenitor as a boring and serrated instrument, like that in so many 
members of the same great order, and that it has since been modi- 
fied but not perfected for iis present purpose, with the poison origi- 
nally adapted for some other object, such as to produce galls, since 
intensified, we can perhaps understand how it is that the use of the 
sting should so often cause the insect's own death: for if on 
the whole the power of stinging be useful to the social commimity, 

M 2 

164 SUMMARY. [Chap. YL 

it will fulfil all the requirements of natural selection, though it may 
cause the death of some few members. If we admire the truly 
wonderful power of scent by which the males of many insects find 
their females, can we admire the production for this single purpose 
of thousands of drones, which are utterly useless to the community 
for any other purpose, and which are ultimately slaughtered by 
their industrious and sterile sisters ? It may be difficult, but we 
ou^^ht to admire the savage instinctive hatred of the queen-bee, 
which urges her to destroy the young queens, her daughters, as soon 
as they are born, or to perish herself in the combat ; for undoubtedly 
this is for the good of the community ; and maternal love or ma- 
ternal hatred, though the latter fortunately is most rare, is all the 
same to the inexorable principle of natural selection. If we admire 
the several ingenious contrivances, by which orchids and many other 
plants are fertilised through insect agency, can we consider as 
equally perfect the elaboration of dense clouds of pollen by our 
fir-trees, so that a few granules may be wafted by chance on to the 
ovules ? 

Summary : the Law of Unity of Type and of the Conditions of 
Existence embraced hy the Theory of Natural Selection. 

We have in this chapter discussed some of the difficulties and 
objections which may be urged against the theory. Many of them 
are serious ; but I think that in the discussion light has been thrown 
on several facts, which on the belief of independent acts of creation 
are utterly obscure. We have seen that species at any one period 
are not indefinitely variable, and are not linked together by a 
multitude of intermediate gradations, partly because the process of 
natural selection is always very slow, and at any one time acts only 
on a few forms; and partly because the very process of natural 
selection implies the continual supplanting and extinction of pre- 
ceding and intermediate gradations. Closely allied species, now 
living on a continuous area, mus.t often have been formed when the 
area was not continuous, and when the conditions of life did not 
insensibly graduate away from one part to another. When two 
varieties are formed in two districts of a continuous area, an inter- 
mediate variety will often be formed, fitted for an intermediate 
sone ; but from reasons assigned, the intermediate variety will 
usually exist in lesser numbers than the two forms which it 
connects ; consequently the two latter, during the course of further 
modification, from existing in greater numbers, will have a great 
advantage over the less numerous intermediate variety, and will 
thus generally succeed in supplanting and exterminating it. 

Chap. VI.] SUMMARY. 165 

We have seen in this chapter how cautious we should be in con- 
dading that the most different habits of life could not graduato 
into each other; that a bat, for instance, could not have been 
formed by natural selection from an animal which at first only- 
glided through the au*. 

We have seen that a species under new conditions of life may 
change its habits ; or it may have diversified habits, with some 
very unlike those of its nearest congeners. Hence we can under- 
stand, bearing in mind that each organic being is trying to live 
wherever it can live, how it has arisen that there are upland geese 
with webbed feet, ground woodpeckers, diving thrushes, and petrels 
with the habits of auks. 

Although the belief that an organ so perfect as the eye could 
have been formed by natural selection, is enough to stagger any 
one ; yet in the case of any organ, if we know of a long series of 
gradations in complexity, each good for its possessor, then, under 
changing conditions of life, there is no logical impossibility in the 
acquirement of any conceivable degree of perfection through natural 
selection. In the cases in which we know of no intermediate or 
transitional states, we should be extremely cautious in concluding 
that none can have existed, for the metamorphoses of many organs 
show what wonderful changes in function are at least possible. For 
instance, a swimbladder has apparently been converted into an air- 
breathing lung. The same organ having performed simultaneously 
very different functions, and then having been in part or in whole 
specialised for one function ; and two distinct organs having per- 
formed at the same time the same function, the one having been 
perfected whilst aided by the other, must often have largely facili- 
tated transitions. 

We have seen that in two beings widely remote from each other 
in the natural scale, organs serving for the same purpose and in 
external appearance closely similar may have been separately and 
independently formed ; but when such organs are closely examined, 
essential differences in their structure can almost always be detected; 
and this naturally follows from the principle of natural selection. 
On the other hand, the common rule throughout nature is infinito 
diversity of structure for gaining the same end ; and this again 
naturally follows from the same great principle. 

In many cases we are far too ignorant to be enabled to assert that 
a part or organ is so unimportant for the welfare of a species, that 
modifications in its structure could not have been slowly accumu- 
lated by means of natural selection. In many other cases, moaifl- 

1G6 SUMMARY. [Chap. VL 

cations are probably the direct result of the laws of variation or oi 
growth, independently of any good having been thus gained. But 
even such structures have often, as we may feel assured, been 
subsequently taken advantage of, and still further modified, for the 
good of species under new conditions of life. We may, also, believe 
that a jjart formerly of high importance has frequently been re- 
tained (as the tail of an aquatic animal by its terrestrial descend- 
ants), though it has become of such small importance that it could 
not, in its present state, have been acquired by means of natural 

Natural selection can produce nothing in one species for the 
exclusive good or injury of another ; though it may well produce 
parts, organs, and excretions highly useful or (;ven indispensable, or 
again highly injurious to another species, but in all cases at the 
same time useful to the possessor. In each well-stocked country 
natural selection acts through the competition of the inhabitants, 
and consequently leads to success in the battle for life, only in 
accordance with the standard of that particular country. Hence 
the inhabitants of one country, generally the smaller one, often 
yield to the inhabitants of another and generally the larger country. 
For in the larger country there will have existed more individuals 
and more diversified forms, and the competition will have been 
severer, and thus the standard of perfection will have been rendered 
higher. Natural selection will not necessarily lead to absolute 
perfection ; nor, as far as we can judge by our limited faculties, can 
absolute perfection be everywhere predicated. 

On the theory of natural selection we can clearly understand the 
full meaning of that old canon in natural history, "Natura non 
facit saltum." This canon, if we look to the present inhabitants 
alone of the world, is not strictly correct ; but if we include all 
those of past times, whether known or unknown, it must on this 
theory be strictly true. 

It is generally acknowledged that all organic beings have been 
formed on two great laws— Unity of Type, and the Conditions of 
Existence. By unity of type is meant that fimdamental agreement 
in structure which we see in organic beings of the same class, and 
which is quite independent of their habits of life. On my theory, 
tmity of type is explained by unity of descent. The expression of 
conditions of existence, so often insisted on by the illustrious 
Cuvier, is fully embraced by the principle of natural selection. For 
natural selection acts by either now adapting the varying parts of 
each being to its organic and inorganic conditions of life ; or by 

Chap. VI.j SUMMARY. 167 

having adapted them during past periods of time : the adaptations 
being aided in many cases by the increased use or disuse of parts, 
being affected by the direct action of the external conditions of 
life, and subjected in all cases to the several laws of growth and 
variation. Hence, in fact, the law of the Conditions of Existence is 
the higher law ; as it includes, through the inheritance of former 
variations and adaptations, that of Unity of Type. 



miscellakeous objections to the theory op natural 


Longevity — Modifications not necessarily simultaneous — Modifications 
apparently of no direct service — Progressive development — Characters 
of small functional importance, the most constant — Supposed incom- 
petence of natural selection to account for the incipient stages of 
useful structures — Causes which interfere with the acquisition through' 
natural selection of useful structures — Gradations of structure with 
changed functions — Widely different organs in members of the same 
class, developed from one and the same source — Reasons for disbeliev- 
ing in great and abrupt modifications. 

I WILL devote this chapter to the consideration of various mis- 
cellaneous objections which have been advanced against my views, 
as some of the previous discussions may thus be made clearer ; but 
it would be useless to discuss all of them, as many have been made 
by writers who have not taken the trouble to understand the 
subject. Thus a distinguished German naturalist has asserted that 
the weakest part of my theory is, that I consider all organic beings 
as imperfect : what I have really said is, that all are not as perfect 
as they might have been in relation to their conditions ; and this is 
ihown to be the case by so many native forms in many quarters of 
the world having yielded their places to intruding foreigners. Nor 
can organic beings, even if they were at any one time perfectly 
adapted to their conditions of life, have remained so, when their 
conditions changed, unless they themselves likewise changed ; and 
no one will dispute that the physical conditions of each country, as 
well as the numbers and kinds of its inhabitants, have undergone 
many mutations. 

A critic has lately insisted, with some parade of mathematical 
accuracy, that longevity is a great advantage to all species, so that 
he who believes in natural selection " must arrange his genealogical 
tree " in such a manner that all the descendants have longer lives 
than their progenitors ! Cannot our critic conceive that a biennial 
plant or one of the lower animals might range into a cold climate. 
and pen'sh there every winter; and yet, owing to advantagcj 


oained through natural selection, survive from year to year by 
means of its seeds or ova? Mr. E. Eay Lankester has recently 
discussed this subject, and he concludes, as far as its extreme com- 
plexity allows him to form a judgment, that longevity is generally 
related to 'he standard of each species in the scale of organisation, 
as well as to the amount of expenditure in reproduction and in 
general activity. And these conditions have, it is probable, been 
Jargely determined through natural selection. 

It has been argued that, as none of the animals and plants of 
Egypt, of which we know anything, have changed during the last 
three or four thousand years, so probably have none in any part 
of the world. But, as Mr. G. H. Lewes has remarked, this line of 
argument proves too much, for the ancient domestic races figured 
on the Egyptian monuments, or embalmed, are closely similar or 
even identical with those now living ; yet all naturalists admit that 
such races have been produced through the modification of their 
original types. The many animals which have remained unchanged 
since the commencement of the glacial period, would have been an 
incomparably stronger case, for these have been exposed to great 
changes of climate and have migrated over great distances ; whereas, 
in Egypt, during the last several thousand years, the conditions of 
life, as far as we know, have remained absolutely uniform. The 
fact of little or no modification having been effected since the 
glacial period would have been of some avail against those who 
believe in an innate and necessary law of development, but is 
powerless against the doctrine of natural selection or the survival 
of the fittest, which implies that when variations or individual 
differences of a beneficial nature happen to arise, these will be 
preserved ; but this will be effected only under certain favourable 

The celebrated palaeontologist, Bronn, at the close of his German 
translation of this work, asks, how, on the principle of natural 
selection, can a variety live side by side with the parent-species ? 
If both have become fitted for slightly different habits of life or 
conditions, they might live together ; and if we lay on one side 
polymorphic species, in which the variability seems to be of a 
peculiar nature, and all mere temporary variations, such as size, 
albinism, &c., the more permanent varieties are generally found, as 
far as I can discover, inhabiting distinct stations, — such as high 
land or low land, dry or moist districts. Moreover, in the case of 
animals which wander much about and cross freely, their varieties 
eeem to be generally confined to distinct regions. 
' Bronn a,' so insists that distinct species never differ from each other 


iu single characters, but in many parts ; and lie asks, how it always 
comes that many parts of the organisation should have been modi- 
fied at the same time through variation and natural selection J 
But there is no necessity for supposing that all the parts of any 
being have been simultaneously modified. The most striking 
modifications, excellently adapted for some purpose, might, as was 
formerly remarked, be acquired by successive variations, if slight, 
first in one part and then in another ; and as they would be trans- 
mitted all together, they would appear to us as if they had been 
simultaneously developed. The best answer, however, to the above 
objection is afforded by those domestic races which have been 
modified, chiefly through man's power of selection, for some special 
pu-rpose. Look at the race and dray horse, or at the greyhound 
and mastiff. Their whole frames and even their mental characteristics 
have been modified ; but if we could trace each step in the history 
of their transformation, — and the latter steps can be traced, — we 
should not see great and simultaneous changes, but first one part 
and then another slightly modified and improved. Even when 
selection has been applied by man to some one character alone, 
— of which our cultivated plants ofier the best instances, — it will 
invariably be found that although this one part, whether it be the 
flower, fruit, or leaves, has been greatly changed, almost all the 
other parts have been slightly modified. This may be attributed 
partly to the principle of correlated growth, and partly to so-called 
spontaneous variation. 

A much more serious objection has been urged by Bronn, and 
recently by Broca, namely, that many characters appear to be of no 
service whatever to their possessors, and therefore cannot have been 
influenced through natural selection. Bronn adduces the length of 
the ears and tails in the different species of hares and mice, — the 
complex folds of enamel in the teeth of many animals, and a 
multitude of analogous cases. With respect to plants, this subject 
has been discussed by Nageli in an admirable essay. He admits that 
natural selection has effected much, but he insists that the families 
of plants differ chiefly from each other in morphological characters, 
which appear to be quite unimportant for the welfare of the species. 
He consequently believes in an innate tendency towards progressive 
and more perfect development. He specifies the arrangement of the 
cells in the tissues, and of the leaves on the axis, as cases in which 
natural selection could not have acted. To these may be added 
the numerical divisions in the parts of the flower, the position 
of the ovules, the shape of the seed, when not of any use for dis- 
eemination, &c. 


There is much force in the above objection. Nevertheless, we 
ought, in the first place, to be extremely cautious in pretending 
to decide what structures now are, or have formerly been, of use to 
each species. In the second place, it should always be borne in 
mind that when one part is modified, so will be other parts, through 
certain dimly seen causes, such as an increased or diminished flow 
of nutriment to a part, mutual pressure, an early developed part 
affecting one subsequently developed, and so forth, — as well as 
through other causes which lead to the many mysterious cases 
of correlation, which we do not in the least understand. These 
agencies may be all grouped together, for the sake of brevity, under 
the expression of the laws of growth. In the third place, we have 
to allow for the direct and definite action of changed conditions of 
life, and for so-called spontaneous variations, in which the nature 
of the conditions apparently plays a quite subordinate part. Bud- 
variations, such as the appearance of a moss-rose on a common 
rose, or of a nectarine on a peach-tree, offer good instances of spon- 
taneous variations ; but even in these cases, if we bear in mind the 
power of a minute drop of poison in producing complex galls, we 
ought not to feel too sure that the above variations are not the 
effect of some local change in the nature of the sap, due to some 
change in the conditions. There must be some efficient cause for 
each slight individual difference, as well as for more strongly 
marked variations which occasionally arise ; and if the unknown 
cause were to act persistently, it is almost certain that all the 
individuals of the species would be similarly modified. 

In the earlier editions of this work I under-rated, as it now seems 
probable, the frequency and importance of modifications due to 
spontaneous variability. But it is impossible to attribute to this 
cause the innumerable structures which are so well adapted to the 
habits of life of each species. I can no more beheve in this, than 
that the well-adapted form of a race^-horse or greyhound, which 
before the principle of selection by man was well understood, excited 
so much surprise in the minds of the older naturalists, can thus be 

It may be worth while to illustrate some of the foregoing remarks. 
With respect to the assumed inutility of various parts and organs, 
H is hardly necessary to observe that even in the higher and best- 
known animals many structures exist, which are so highly developed 
that no one doubts that they are of importance, yet their use has 
not been, or has only recently been, ascertained. As Bronn gives 
the length of the ears and tail in the several species of mice as 
instances, though trifling ones, of differences in structure which can 


be of no special use, I may mention that, according to Dr. Schobl, 
the external ears of the common mouse are supplied in an extra- 
ordinary manner with nerves, so that they no doubt serve as tactile 
organs ; hence the length of the ears can hardly be quite unim- 
portant. Wo shall, also, presently see that the tail is a highly 
useful prehensile organ to some of the species ; and its use would 
be mnch influenced by its length. 

With respect to plants, to which on account of Nageli's essay 
I shall confine myself in the following remarks, it will be admitted 
that the flowers of orchids present a multitude of curious structures, 
which a few years ago would have been considered as mere morpho- 
locrical differences without any special function ; but they are now 
known to be of the highest importance for the fertilisation of the 
species through the aid of insects, and have probably been gained 
through natural selection. No one until lately would have imagined 
that in dimorphic and trimorphic plants the different lengths of the 
stamens and pistils, and their arrangement, could have been of any 
service, but now we know this to be the case. 

In certain whole groups of plants the ovules stand erect, and in 
others they are suspended ; and within the same ovarium of some 
few plants, one ovule holds the former and a second ovule the latter 
position. These positions seem at first purely morphological, or 
of no physiological signification ; but Dr. Hooker informs me that 
within the same ovarium, the upper ovules alone in some cases, 
and in other cases the lower ones alone are fertilised ; and he 
suggests that this probably depends on the direction in which the 
pollen-tubes enter the ovarium. If so, the position of the ovules, 
even when one is erect and the other suspended within the same 
ovarium, would follow from the selection of any slight deviations in 
position which favoured their fertilisation, and the production of seed. 

Several plants belonging to distinct orders habitually produce 
flowers of two kinds, — the one open of the ordinary structure, the 
other closed and imperfect. These two kinds of flowers sometimes 
differ wonderfully in structure, yet may be seen to graduate into 
each other on the same plant. The ordinary and open flowers can 
be intercrossed ; and the benefits which certainly are derived from 
this process are thus secured. The closed and imperfect flowers are, 
however, manifestly of high importance, as they yield with the 
utmost safety a large stock of seed, with the expenditure of wonder- 
fully little pollen. The two kinds of flowers often differ much, aa 
just stated, in structure. The petals in the imperfect flowers almost 
always consist of mere rudiments, and the pollen-grains are reduced 
in diameter. In Ononis columna3 five of the alternate stamens aro 


rudimentary ; and in some species of Viola three stamens are in 
this state, two retaining their proper function, but being of very 
small size. In six out of thirty of the closed flowers in an Indian 
violet (name unknown, for the plants have never produced with me 
perfect flowers), the sepals are reduced from the normal number 
of five to three. In one section of the MalpighiacejB the closed 
flowers, according to A. de Jussieu, are still further modified, for 
the five stamens which stand opposite to the sepals are all aborted, 
a sixth stamen standing opposite to a petal being alone developed ; 
and this stamen is not present in the ordinary flowers of these 
species ; the style is aborted ; and the ovaria are reduced from three 
to two. Now although natural selection may well have had the 
power to prevent some of the flowers from expanding, and to reduce 
the amount of pollen, when rendered by the closure of the flowers 
superfluous, yet hardly any of the above special modifications can 
have been thus determined, but must have followed from the laws of 
growth, including the functional inactivity of parts, during the pro- 
gress of the reduction of the pollen and the closure of the flowers. 

It is so necessary to appreciate the important efiects of the laws 
of growth, that I will give some additional cases of another kind, 
namely of differences in the same part or organ, due to differences 
in relative position ou the same plant. In the Spanish chestnut, 
and in certain fir-trees, the angles of divergence of the leaves differ, 
according to Schacht, in the nearly horizontal and in the upright 
branches. In the common rue and some other plants, one flower, 
usually the central or terminal one, opens first, and has five sepals 
and petals, and five divisions to the ovarium ; whilst all the other 
flowers on the plant are tetramerous. In the British Adoxa the 
uppermost flower generally has two calyx-lobes with the other 
organs tetramerous, whilst the surrounding flowers generally have 
three calyx-lobes with the other organs pentamerous. In many 
Compositse and Umbelliferse (and in some other plants) the circum- 
ferential flowers have their corollas much more developed than those 
of the centre ; and this seems often connected with the abortion of the 
reproductive organs. It is a more curious fact, previously referred 
to, that the achenes or seeds of the circumference and centre 
sometimes differ greatly in form, colour, and other characters. In 
Carthamus and some other Compositse the central achenes alone 
are furnished with a pappus ; and in Hyoseris the same head yields 
achenes of three different forms. In certain Umbelliferse the ex- 
terior seeds, according to Tausch, are orthospermous, and the central 
one coslospermous, and this is a character which was considered by 
De Candolle to be in other species of the highest systematic im- 


portance. Prof. Braun mentions a Fumariaceous genus, in wliich 
the flowers in the lower part of the spike bear oval, ribbed, one- 
seeded nutlets ; and in the upper part of the spike, lanceolate, two- 
valved, and two-seeded siliques. In these several cases, with the 
exception of that of the well developed ray-florets, which are of service 
in making the flowers conspicuous to insects, natural selection can- 
not, as far as we can judge, have come into play, or only in a quite 
subordinate manner. All these modifications follow from the relative 
position and inter-action of the parts ; and it can hardly be doubted 
that if all the flowers and leaves on the same plant had been sub- 
jected to the same external and internal condition, as are the flowers 
and leaves in certain positions, all would have been modified in the 
same manner. 

In numerous other cases we find modifications of structure, which 
are considered by botanists to be generally of a highly import- 
ant nature, affecting only some of the flowers on the same plant, 
or occurring on distinct plants, which grow close together under the 
same conditions. As these variations seem of no special use tc 
the plants, they cannot have been influenced by natural selection. 
Of their cause we are quite ignorant; we cannot even attribute 
them, as in the last class of cases, to any proximate agency, such 
as relative position. I will give only a few instances. It is so 
common to observe on the same plant, flowers indifferently tetra- 
merous, pentamerous, &c., that I need not give examples; but as 
numerical variations are comparatively rare when the parts are 
few, I may mention that, according to De Candolle, the flowers or 
Papaver bracteatum offer either two sepals with four petals (which 
is the common type with poppies), or three sepals with six petals. 
The manner in which the petals are folded in the bud is in most 
groups a very constant morphological character ; but Professor Asa 
Gray states that with some species of Mimulus, the aestivation is 
almost as frequently that of the Ehinanthidea3 as of the Antirrhi- 
nidese, to which latter tribe the genus belongs. Aug. St. Hilaire 
gives the following cases : the genus Zanthoxylon belongs to a 
division of the Eutacese with a single ovary, but in some species 
flowers may be found on the same plant, and even in the same 
panicle, with either one or two ovaries. In Helianthemum the 
capsule has been described as unilocular or 3-locular; and in 
H. mutabile, "Une lame, jplus ou moins large, s'etend entre le 
pericarpe et le placenta." In the flowers of Saponaria officinalis, 
Dr. Masters has observed instances of both marginal and free central 
placentation. Lastly, St. Hilaire found towards the southern ex- 
treme of the range of Gomphia ole^formis two forms which he did 


not at first doubt were distinct species, but lie subsequently saw 
them growing on the same bush ; and he then adds, " Voilk done 
dans im mgme individu des loges et un style qui se rattachent 
tantot h un axe verticale et tantot k un gynobase." 

We thus see that with plants many morphological changes may 
be attributed to the laws of growth and the inter-action of parts, 
independently of natural selection. But with respect to Nageli's 
doctrine of an innate tendency towards perfection or progressive 
development, can it be said in the case of these strongly pro- 
nounced variations, that the plants have been caught in the act of 
progressing towards a higher state of development ? On the con • 
trary, I should infer from the mere fact of the parts in question 
differing or varying greatly on the same plant, that such modi- 
fications were of extremely small importance to the plants them- 
selves, of whatever importance they may generally be to us for 
our classifications. The acquisition of a useless part can hardly 
be said to raise an organism in the natural scale ; and in the case 
of the imperfect, closed flowers above described, if any new prin- 
ciple has to be invoked, it must be one of retrogression rather than 
of progression ; and so it must be with many parasitic and degraded 
animals. We are ignorant of the exciting cause of the above 
specified modifications ; but if the unknown cause were to act 
almost uniformly for a length of time, we may infer that the result 
would be almost uniform ; and in this case all the individuals of the 
species would be modified in the same manner. 

From the fact of the above characters being unimportant for the 
welfare of the species, any slight variations which occurred in them 
would not have been accumulated and augmented through natural 
selection. A structure which has been developed through loug- 
continued selection, when it ceases to be of service to a species, 
generally becomes variable, as we see with rudimentary organs ; for 
it will no longer be regulated by this same power of selection. 
But when, from the nature of the organism and of the conditions, 
modifications have been induced which are unimportant for the 
welfare of the species, they may be, and apparently often have 
been, transmitted in nearly the same state to numerous, otherwise 
modified, descendants. It cannot have been of much importance 
to the greater number of mammals, birds, or reptiles, whether they 
were clothed with hair, feathers, or scales ; yet hair has been trans- 
mitted to almost all mammals, feathers to all birds, and scales to 
all true reptiles. A structure, whatever it may be, which is com- 
mon to many allied forms, is ranked by us as of high systematic 
importance, and consequently is often assumed to be of high vi tal 


importance to the species. Thus, as I am inclined to believe, 
morphological diflferences, which we consider as important — snch aji 
the arrangement of the leaves, the divisions of the flower or of the 
ovarimn, the position of the ovules, &c. — first appeared in many 
cases as fluctuating variations, which sooner or later became con- 
stant through the nature of the organism and of the surrounding 
conditions, as well as through the intercrossing of distinct indivi- 
duals, but not through natural selection ; for as these morphological 
characters do not affect the welfare of the species, any slight devia- 
tions in them could not have been governed or accumulated through 
this latter agency. It is a strange result which we thus arrive at, 
namely that characters of slight vital importance to the species, 
are the most important to the systematist ; but, as we shall here- 
after see when we treat of the genetic principle of classification, 
this is by no means so paradoxical as it may at first appear. 

Although we have no good evidence of the existence in organic 
beings of an innate tendency towards progressive development, yet 
this necessarily follows, as I have attempted to show in the fourth 
chapter, through the continued action of natural selection. For the 
best definition which has ever been given of a high standard of 
organisation, is the degree to which the parts have been specialised 
or differentiated ; and natural selection tends towards this end, inas- 
much as the parts are thus enabled to perform their functions more 

A distinguished zoologist, Mr. St. George Mivart, has recently 
collected all the objections which have ever been advanced by 
myself and others against the theory of natural selection, as pro- 
pounded by Mr. Wallace and myself, and has illustrated them with 
admirable art and force. When thus marshalled, they make a 
formidable array ; and as it forms no part of Mr. Mivart's plan to 
give the various facts and considerations opposed to his conclusions, 
no slight effort of reason and memory is left to the reader, who may 
wish to weigh the evidence on both sides. When discussing special 
cases, Mr. Mivart passes over the effects of the increased use and 
disuse of parts, which I have always maintained to be highly im- 
portant, and have treated in my * Variation under Domestication ' 
at greater length than, as I believe, any other writer. Ho likewise 
often assumes that I attribute nothing to variation, independently 
of natural selection, whereas in the work just referred to I have 
collected a greater number of well-established cases than can be 
found in any other work known to me. My judgment may not be 
trnatworthy, but after reading with care Mr. Mivart's book» and 


comparing each section with what I have said on the same heacL 
I never before felt so strongly convinced of the general truth of the 
conclusions here arrived at, subject, of course, in so intricate a 
subject, to much partial error. 

All Mr. Mivart's objections will be, or have been, considered in 
the present volume. The one new point which appears to have 
struck many readers is, " that natural selection is incompetent to 
account for the incipient stages of useful structures." This subject 
is intimately connected with that of the gradation of characters, 
often accompanied by a change of function, — for instance, the con- 
version of a swim-bladder into lungs, — points which were discussed 
in the last chapter under two headings. Nevertheless, I will here 
consider in some detail several of the cases advanced by Mr. Mivart, 
selecting those which are the most illustrative, as want of space 
prevents me from considering all. 

The giraffe, by its lofty stature, much elongated neck, fore-legs, 
head and tongue, has its whole frame beautifully adapted for 
browsing on the higher branches of trees. It can thus obtain food 
beyond the reach of the other Ungulata or hoofed animals inhabiting 
the same country ; and this must be a great advantage to it during 
dearths. The Niata cattle in S. America show us how small a 
difference in structure may make, during such periods, a great differ- 
ence in preserving an animal's life. These cattle can browse as well 
as others on grass, but from the projection of the lower jaw they 
cannot, during the often recurrent droughts, browse on the twigs 
of trees, reeds, &c,, to which food the common cattle and horses 
are then driven ; so that at these times the Niatas perish, if not fed 
by their owners. Before coming to Mr. Mivart's objections, it may 
be well to explain once again how natural selection will act in all 
ordinary cases. Man has modified some of his animals, without 
necessarily having attended to special points of structure, by simply 
preserving and breeding from the fleetest individuals, as with the 
race-horse and greyhound, or as with the game-cock, by breeding 
from the victorious birds. So under nature with the nascent giraffe, 
the individuals which were the highest browsers and were able 
during dearths to reach even an inch or two above the others, will 
often have been preserved; for they will have roamed over the 
whole country in search of food. That the individuals of the same 
species often differ slightly in the relative lengths of all their parts 
may be seen in many works of natural history, in which careful 
measurements are given. These shght proportional differences, due 
to the laws of growth and variation, are not of the slightest use or 
importance to most species. But it will have been otherwise with 


the nascent giraffe, considering its probable habits of Ufe ; for those 
individuals which had some one part or several parts of their bodies 
rather more elongated than usual, would gen-erally have survived. 
These will have intercrossed and left offspring, either inheriting the 
same bodily pecuUarities, or with a tendency to vary again in the 
same manner ; whilst the individuals, less favoured in the same 
respects, will have been the most liable to perish. 

We here see that there is no need to separate single pairs, as man 
does, when he methodically improves a breed : natural selection will 
preserve and thus separate all the superior individuals, allowing 
them freely to intercross, and will destroy all the inferior indivi- 
duals. By this process long-continued, which exactly corresponds 
with what I have called unconscious selection by man, combined no 
doubt in a most important manner with the inherited effects of the 
increased use of parts, it seems to me almost certain that an ordinary 
hoofed quadruped might be converted into a giraffe. 

To this conclusion Mr. Mivart brings forward two objections. 
One is that the increased size of the body would obviously require 
an increased supply of food, and he considers it as "very problemati- 
cal whether the disadvantages thence arising would not, in times of 
scarcity, more than counterbalance the advantages." But as the 
giraffe does actually e"xist in large numbers in S. Africa, and as 
some of the largest antelopes in the world, taller than an ox, abound 
there, why should we doubt that, as far as size is concerned, inter- 
mediate gradations could formerly have existed there, subjected as 
now to severe dearths. Assuredly the being able to reach, at each 
stage of increased size, to a supply of food, left untouched by the 
other hoofed quadrupeds of the country, would have been of some 
advantage to the nascent giraffe. Nor must we overlook the fact, 
that increased bulk would act as a protection against almost all 
beasts of prey excepting the lion ; and against this animal, its tall 
neck, — and the taller the better, — would, as Mr. Chauncey Wright 
iias remarked, serve as a watch-tower. It is from this cause, as Sir 
S. Baker remarks, that no animal is more difficult to stalk than the 
giraffe. This animal also uses its long neck as a means of offence 
or defence, by violently swinging its head armed with stump-like 
horns. The preservation of each species can rarely be determined 
by any one advantage, but by the union of all, great and small. 

Mr. Mivart then asks (and this is his second objection), if natural 
selection be so potent, and if high browsing be so great an advan- 
tage, why has not any other hoofed quadruped acquired a long neck 
and lofty statui'e, besides the giraffe, and, in a lesser degree, the 
camel, guanaco, and macrauchenia ? Or, again, why has not any 


member of the group acquired a long proboscis ? With respect to 
S. Africa, which was formerly inhabited by numerous herds of the 
giraffe, the answer is not difficult, and can best be given by an 
illustration. In every meadow in England in which trees grow, 
we see the lower branches trimmed or planed to an exact level by 
the browsing of the horses or cattle ; and what advantage would it 
be, for instance, to sheep, if kept there, to acquire slightly longer 
necks? In every district some one kind of animal will almost 
certainly be able to browse higher than the others ; and it is almost 
equally certain that this one kind alone could have its neck 
elongated for this purpose, through natural selection and the effects 
of increased use. In S. Africa tjie competition for browsing on the 
higher branches of the acacias and other trees must be between 
giraffe and giraffe, and not with the other ungulate animals. 

Why, in other quarters of the world, various animals belonging 
to this same order have not acquired either an elongated neck 
or a proboscis, cannot be distinctly answered; but it is as un- 
reasonable to expect a distinct answer to such a question, as 
why some event in the history of mankind did not occur in one 
country, whilst it did in another. We are ignorant with respect to 
the conditions which determine the numbers and range of each 
species ; and we ca^nnot even conjecture what changes of structure 
would be favourable to its increase in some new country. We can, 
however, see in a general manner that various causes might have 
interfered with the development of a long neck or proboscis. To 
reach the foliage at a considerable height (without climbing, for 
which hoofed animals are singularly ill-constructed) implies greatly 
increased bulk of body; and we know that some areas support 
singularly few large quadrupeds, for instance S. America, though it 
is so luxuriant ; whilst S. Africa abounds with them to an un- 
paralleled degree. Why this should be so, we do not know ; nor 
why the later tertiary periods should have been much more favour- 
able for their existence than the present time. Whatever the 
fauses may have been, we can see that certain districts and times 
would have been much more favourable than others for the develop- 
ment of so large a quadruped as the giraffe. 

In order that an animal should acquire some structure specially 
and largely developed, it is almost indispensable that several other 
parts should be modified and co-adapted. Although every part of 
the body varies slightly, it does not follow that the necessary parts 
should always vary in the right direction and to the right degree. 
With the different species of our domesticated animals we kno^ 
that the parts vary in a different manner and degree; and that 

N 2 


some species are much more variable than, others. Even if the 
fitting variations did arise, it does not follow that natural selection 
would be able to act on them, and produce a structure which ap- 
parently would be beneficial to the species. For instance, if the 
number of individuals existing in a country is determined chiefly 
through destruction by beasts of prey,— by external or internal 
parasites, &c., — as seems often to be the case, then natural selection 
will be able to do little, or will be greatly retarded, in modifying 
any particular structure for obtaining food. Lastly, natural selec- 
tion is a slow process, and the same favourable conditions must 
long endure in order that any marked effect should thus be pro- 
duced. Except by assigning such general and vague reasons, we 
cannot explain why, in many quarters of the world, hoofed quadru- 
peds have not acquired much elongated necks or other means for 
browsing on the higher branches of trees. 

Objections of the same nature as the foregoing have been advanced 
by many writers. In each ca&e various causes, besides the general 
ones just indicated, have probably interfered with the acquisition 
through natural selection of structures, which it is thought would be 
beneficial to certain species. One writer asks, why has not the 
ostrich acquired the power of flight ? But a moment's reflection 
will show what an enormous supply of food would be necessary to 
give to this bird of the desert force to move its huge body through 
the air. Oceanic islands are inhabited by bats and seals, but by no 
terrestrial mammals ; yet as some of these bats are peculiar species, 
they must have long inhabited their present homes. Therefore 
Sir C. Lyell asks, and assigns certain reasons in answer, why have 
not seals and bats given birth on such islands to forms fitted to 
live on the land ? But seals would necessarily be first converted 
into terrestrial carnivorous animals of considerable size, and bats into 
terrestrial insectivorous animals ; for the former there would be 
no prey ; for the bats ground-insects would serve as food, but 
these would already be largely preyed on by the reptiles or birds, 
which first colonise and abound on most oceanic islands. Gradations 
of structure, with each stage beneficial to a changing species, will 
be favoured only under certain peculiar conditions. A strictly 
terrestrial animal, by occasionally hunting for food in shallow 
water, then in streams or lakes, might at last be converted into an 
animal so thoroughly aquatic as to brave the open ocean. But seals 
would not find on oceanic islands the conditions favourable to their 
gradual reconversion into a terrestrial form. Bats, as formerly 
shown, probably acquired their wings by at first gliding through 
the air from tree to tree, like the so-called fiying-squirreLst 


for the sake of escaping from their enemies, or for aroiding falls ; 
but when tho power of true flight had once been acquired, it would 
never be reconverted back, at least for the above purposes, into the 
kss efficient power of gliding through the air. Bats might, indeed, 
like many birds, have had their wings greatly reduced in size, or 
completely lost, through disuse; but in this case it would be 
necessary that they should first have acquired the power of running 
quickly on the ground, by the aid of their hind legs alone, so as to 
compete with birds or other ground animals; and for such a change 
a bat seems singularly ill-fitted. These conjectural remarks have 
been made merely to show that a transition of structure, with each 
step beneficial, is a highly complex affair ; and that there is nothing 
strange in a transition not having occurred in any particular case. 

Lastly, more than one writer has asked, why have some animals 
had their mental povf er? more highly developed than others, as such 
development would be advantageous to all? Why have not apes 
acquired the intellectual powers of man ? Various causes could be 
assigned ; but as they are conjectural, and their relative probability 
cannot be weighed, it would be useless to give them. A definite 
answer to the latter question ought not to be expected, seeing that 
no one can solve the simpler problem why, of two races of savages, 
one has risen higher in the scale of civilisation than the other ; and 
this apparently implies increased brain-power. 

We will return to Mr. Mivart's other objections. Insects often 
resemble for the sake of protection various objects, such as green or 
decayed leaves, dead twigs, bits of lichen, flowers, spines, excrement 
of birds, and living insects ; but to this latter point I shall here- 
after recur. The resemblance is often wonderfully close, and is not 
confined to colour, but extends to form, and even to the manner in 
which the insects hold themselves. The caterpillars which project 
motionless like dead twigs from the bushes on which they feed, 
offer an excellent instance of a resemblance of this kind. The 
cases of the imitation of such objects as the excrement of birds, are 
rare and exceptional. On this head, Mr. Mivart remarks, "As, 
according to Mr. Darwin's theory, there is a constant tendency to 
indefinite variation, and as the minute incipient variations will be 
in all directions, they must tend to neutralize each other, and at 
first to form such unstable modifications that it is difficult, if not 
impossible, to see how such indefinite oscillations of infinitesimal 
beginnings can ever build up a sufficiently appreciable resemblance 
to a leaf, bamboo, or other object, for Natural Seliction to seize 
upon and perpetuate." 


But in all the foregoing cases the insects in their original state 
no doubt presented some rude and accidental resemblance to an 
object commonly found in the stations frequented by them. Nor 
is this at all improbable, considering the almost infinite number of 
surrounding objects and the diversity in form and colour of the 
hosts of insects which exist. As some rude resemblance is neces- 
sary for the first start, we can understand how it is that the larger 
and higher animals do not (with the exception, as far as I know, of 
one fish) resemble for the sake of protection special objects, but 
only the surface which commonly surrounds them, and this chiefly 
in colour. Assuming that an insect originally happened to resemble 
in some degree a dead twig or a decayed leaf, and that it varied 
slightly in many ways, then all the variations which rendered the 
insect at all more like any such object, and thus favoured its escape, 
would be preserved, whilst other variations would be neglected and 
ultimately lost ; or, if they rendered the insect at all less like the 
imitated object, they would be eliminated. There would indeed be 
force in Mr. Mivart's objection, if we were to attempt to account 
for the above resemblances, independently of natural selection, 
through mere fluctuating variability ; but as the case stands there 
is none. 

Nor can I see any force in Mr. Mivart's difficulty with respect to 
" the last touches of perfection in the mimicry ;" as in the case 
given by Mr. Wallace, of a walking-stick insect (Ceroxylus 
iaceratus), which resembles "a stick grown over by a creeping 
moss or jungermannia." So close was this resemblance, that a 
native Dyak maintained that the foliaceous excrescences were really 
moss. Insects are preyed on by birds and other enemies, whose 
sight is probably sharper than ours, and every grade in resemblance 
which aided an insect to escape notice or detection, would tend 
towards its preservation ; and the more perfect the resemblance so 
much the better for the insect. Considering the nature of the differ- 
ences between the species in the group which includes the above 
Ceroxylus, there is nothing improbable in this insect having varied 
in the irregularities on its surface, and in these having become more 
or less green-coloured ; for in every group the characters which 
differ in the several species are the most apt to vary, whilst the 
generic characters, or those common to all the species, are the most 

The Greenland whale is one of the most wonderful animals in the 
world, and the baleen, or whale-bone, one of its greatest pecu- 
liarities. The baleen consists of a row, on each side, of the upper 


jaw, of about 300 plates or laminse, which stand close together 
transversely to the longer axis of the mouth. Within the main row 
there are some subsidiary rows. The extremities and inner margins 
of all the plates are frayed into stiff bristles, which clothe the whole 
gigantic palate, and serve to strain or sift the water, and thus to 
secure the minute prey on which these great animals subsist. The 
middle and longest lamina in the Greenland whale is ten, twelve, or 
even fifteen feet in length ; but in the different species of Cetaceans 
there are gradations in length; the middle lamina being in one 
species, according to Scoresby, four feet, in another three, in 
another eighteen inches, and in the Balsenoptera rostrata only about 
nine inches in length. The quality of the whale-bone also differs in 
the different species. 

With respect to the baleen, Mr. Mivart remarks that if it " had 
once attained such a size and development as to be at all useful, 
then its preservation and augmentation within serviceable limits 
would be promoted by natural selection alone. But how to obtain 
the beginning of such useful development ? " In answer, it may 
be asked, why should not the early progenitors of the whales with, 
baleen have possessed a mouth constructed something like the 
lamellated beak of a duck ? Ducks, like whales, subsist by sifting 
the mud and water; and the family has sometimes been called 
CrihlatoreSj or sifters. I hope that I may not be misconstrued inta 
saying that the progenitors of whales did actually possess mouths 
lamellated like the beak of a duck. I wish only to show that this 
is not incredible, and that the immense plates of baleen in the 
Greenland whale might have been developed from such lamellae by 
finely graduated steps, each of service to its possessor. 

The beak of a shoveller-duck (Spatula clypeata) is a more beau- 
tiful and complex structure than the mouth of a whale. The upper 
mandible is furnished on each side (in the specimen examined by 
me) with a row or comb formed of 188 thin, elastic lamellse, 
obliquely bevelled so as to be pointed, and placed transversely to 
the longer axis of the mouth. They arise from the palate, and are 
attached by flexible membrane to the sides of the mandible. Those 
standing towards the middle are the longest, being about one-third 
of an inch in length, and they project * 14 of an inch beneath the 
edge. At their bases there is a short subsidiary row of obliquely 
transverse lamellae. In these several respects they resemble the 
plates of baleen in the mouth of a whale. But towards the 
extremity of the beak they differ much, as they project inwards, 
instead of straight downwards. The entire head of the shoveller, 
though incomparably less bulky, is about one-eighteenth of the 


lerLgth of the heaxi of a moderately large Balasnoptera rostrata, in 
which species the baleen is only nine inches long ; so that if we 
were to make the head of the shoveller as long as that of the 
Baleenoptera, the lamellas would be six inches in length, — that is, 
two-thirds of the length of the baleen in this species of whale. The 
lower mandible of the shoveller-dnck is furnished with lamellae of 
equal length with those above, but finer ; and in being thus fur- 
nished it differs conspicuously from the lower jaw of a whale, which 
is destitute of baleen. On the other hand, the extremities of these 
lower lamellse are frayed into fine bristly points, so that they thus 
curiously resemble the plates of baleen. In the genus Prion, a 
member of the distinct family of the Petrels, the upper mandible 
alone is furnished with lamellge, which are well developed and 
project beneath the margin ; so that the beak of this bird resembles 
in this respect the mouth of a whale. 

From the highly developed structure of the shoveller's beak we 
may proceed (as I have learnt from information and specimens sent 
to me by Mr. Salvin), without any great break, as far as fitness for 
sifting is concerned, through the beak of the Merganetta armata, and 
in some respects through that of the Aix sponsa, to the beak of the 
common duck. In this latter species, the lamellse are much coarser 
than in the shoveller, and are firmly attached to the sides of the 
mandible ; they are only about 50 in number on each side, and do not 
project at all beneath the margin. They are square-topped, and aro 
edged with translucent hardish tissue, as if for crushing food. The 
edges of the lower mandible are crossed by numerous fine ridges, 
which project very little. Although the beak is thus very inferior 
as a sifter to that of the shoveller, yet this bird, as every one knows, 
constantly uses it for this purpose. There are other species, as I 
hear from Mr. Salvin, in which the lamellae are considerably less 
developed than in the common duck ; but I do not know whether 
they use their beaks for sifting the water. 

Turning to another group of the same family. In the Egyptian 
goose (Chenalopex) the beak closely resembles that of the common 
duck ; but the lamellaB are not so numerous, nor so distinct from 
each other, nor do they project so much inwards ; yet this goose, is 
I am informed by Mr. E. Bartlett, " uses its bill like a duck by 
throwing the water out at the corners." Its chief food, howeA-er, is 
grass, which it crops like the common goose. In this latter bird^ 
the lamella3 of the upper mandible are much coarser than in the 
common duck, almost confluent, about 27 in number on each 
side, and terminating upwards in teeth-like knobs. The palate is 
also covered with hard rounded knobs. The edges of the lower 


mandible are serrated with teeth much more prominent, coarser, 
and sharper than in the duck. The common goose does not sift the 
water, but uses its beak exclusively for tearing or cutting herbage, 
for which purpose it is so well fitted, that it can crop grass closer 
than almost any other animal. There are other species of geese, as 
I hea;r from Mr. Bartlett, in which the lamellas are less developed 
than in the common goose. 

We thus see that a member of the duck family, with a beak 
constructed like that of the common goose and adapted solely for 
grazing, or even a member with a beak having less well-developed 
lamellae, might be converted by small changes into a species like 
the Egyptian goose, — this into one like the common duck, — and, 
lastly, into one like the shoveller, provided with a beak almost 
exclusively adapted for sifting the water ; for this bird could hardly 
use any part of its beak, except the hooked tip, for seizing or tearing 
solid food. The beak of a goose, as I may add, might also be con- 
verted by small changes into one provided with prominent, recurved 
teeth, like those of the Merganser (a member of the same family), 
serving for the widely different purpose of securing live fish. 

Eeturning to the whales. The Hyperoodon bidens is destitute of 
true teeth in an efficient condition, but its palate is roughened, 
according to Lacepede, with small, unequal, hard points of horn. 
There is, therefore, nothing improbable in supposing that some 
early Cetacean form was provided with similar points of horn on the 
palate, but rather more regularly placed, and which, like the knobs 
on the beak of the goose, aided it in seizing or tearing its food. If 
so, it will hardly be denied that the points might have been con- 
verted through variation and natural selection into lamellae as well- 
developed as those of the Egyptian goose, in which case they would 
have been used both for seizing objects and for sifting the water ; 
then into lamellae like those of the domestic duck ; and so onwards, 
until they became as well constructed as those of the shoveller, in 
which case they would have served exclusively as a sifting appa- 
ratus. From this stage, in which the lamellae would be two-thirds 
of the length of the plates of baleen in the Balaenoptera rostrata, 
gradations, which may be observed in still-existing Cetaceans, lead 
us onwards to the enormous plates of baleen in the Greenland 
whale. Nor is there the least reason to doubt that each step in 
this scale might have been as serviceable to certain ancient Ceta- 
ceans, v;ith the functions of the parts slowly changing during the 
progress of development, as are the gradations in the beaks oi 
the differ3nt existing members of the duck-family. We should 
bear in mind that each species of duck is subjected to a severe 


struggle for existence, and that the structure of every part of its 
frame must be well adapted to its conditions of life. 

The Pleuronectidse, or Flat-fish, are remarkable for their asym- 
metrical bodies. They rest on one side, — in the greater number of 
species on the left, but in some on the right side ; and occasionallj 
reversed adult specimens occur. The lower, or resting-surface, re- 
sembles at first sight the ventral surface of an ordinary fish : it is of 
a white colour, less developed in many ways than the upper side, 
with the lateral fins often of smaller size. But the eyes offer the 
most remarkable peculiarity ; for they are both placed on the upper 
side of the head. During early youth, however, they stand oppo- 
site to each other, and the whole body is then symmetrical, with 
both sides equally coloured. Soon the eye proper to the lower side 
begins to glide slowly round the head to the upper side ; but does 
not pass right through the skull, as was formerly thought to be 
the case. It is obvious that unless the lower eye did thus travel 
round, it could not be used by the fish whilst lying in its habitual 
position on ono side. The lower eye would, also, have been liable 
to be abraded by the sandy bottom. That the Pleuronectidse are 
admirably adapted by their flattened and asymmetrical structure 
for their habits of life, is manifest from several species, such as 
soles, flounders, &c., being extremely common. The chief ad- 
vantages thus gained seem to be protection from their enemies, 
and facility for feeding on the ground. The difi"erent members, 
however, of the family present, as Schiodte remarks, " a long series 
of forms exhibiting a gradual transition from Hippoglossus pinguis, 
which does not in any considerable degree alter the shape in which 
it leaves the ovum, to the soles, which are entirely thrown to one 

Mr. Mivart has taken up this case, and remarks that a sudden 
spontaneous transformation in the position of the eyes is hardly 
conceivable, in which I quite agree with him. He then adds : " if 
the transit was gradual, then how such transit of one eye a minute 
traction of the journey towards the other side of the head could 
benefit the individual is, indeed, far from clear. It seems, even, 
that such an incipient transformation must rather have been inju- 
rious." But he might have found an answer to this objection in 
the excellent observations published in 1867 by Malm. The 
Pleuronectida3, whilst very young; and still symmetrical, with their 
eyes standing on opposite sides of the head, cannot long retain 
a vertical position, owing to the excessive depth of their bodies, the 
small Bize of their lateral fins, and to their being destitute of a 


swimbladder. Hence soon growing tired, they fall to the bottom 
on one side. Whilst thus at rest they often twist, as Malm 
observed, the lower eye upwards, to see above them ; and they 
do this so vigorously that the eye is pressed hard against the 
upper part of the orbit. The forehead between the eyes conse- 
quently becomes, as could be plainly seen, temporarily contracted 
in breadth. On one occasion Malm saw a young fish raise and 
depress the lower eye through an angular distance of about seventy 

AYe should remember that the skull at this early age is cartila- 
ginous and flexible, so that it readily yields to muscular action. 
It is also known with the higher animals, even after early youth, 
that the skull yields and is altered in shape, if the skin or muscles 
be permanently contracted through disease or some accident. With 
long-eared rabbits, if one ear lops forwards and downwards, its 
weight drags forward all the bones of the skull on the same side, of 
which I have given a figure. Malm states that the newly-hatched 
young of perches, salmon, and several other symmetrical fishes, 
have the habit of occasionally resting on one side at the bottom ; 
and he has observed that they often then strain their lower eyes 
so as to look upwards ; and their skulls are thus rendered rather 
crooked. These fishes, however, are soon able to hold themselves 
in a vertical position, and no permanent effect is thus produced. 
With the Pleuronectidse, on the other hand, the older they grow 
the more habitually they rest on one side, owing to the increasing 
flatness of their bodies, and a permanent efi"ect is thus produced on 
the form of the head, and on the position of the eyes. Judging 
ffom analogy, the tendency to distortion would no doubt be 
increased through the principle of inheritance. Schiodte believes, 
in opposition to some other naturalists, that the Pleuronectidje are 
not quite symmetrical even in the embryo ; and if this be so, we 
could understand how it is that certain species, whilst young, 
habitually fall over and rest on the left side, and other species on 
the right side. Malm adds, in confirmation of the above view, that 
the adult Trachypterus arcticus, which is not a member of the 
Pleuronectidse, rests on its left side at the bottom, and swims 
diagonally through the water ; and in this fish, the two sides of the 
head are said to be somewhat dissimilar. Our great authority on 
Fishes, Dr. Giinther, concludes his abstract of Malm's paper, by 
remarking that " the author gives a very simple explanation of the 
abnormal condition of the Pleuronectoids." 

We thus see that the first stages of the transit of the eyo from 
gho cide of the head to the other, which Mr. Mivart considers woiild 


be injurious, may be attributed to the habit, no doubt bene- 
ficial to the individual and to the species, of endeavouring to 
look upwards with both eyes, whilst resting on one side at the 
bottom. We may also attribute to the inherited effects of use 
the fact of the mouth in several kinds of flat-fish being bent 
towards the lower surface, with the jaw bones stronger and more 
effective on this, the eyeless side of the head, than on the other, 
for the sake, as Dr. Traquair supposes, of feeding with ease on the 
ground. Disuse, on the other hand, will account for the less deve- 
loped condition of the whole inferior half of the body, including the 
lateral fins ; though Yarrell thinks that the reduced size of these 
fins is advantageous to the fish, as " there is so much less room for 
their action, than with the larger fins above." Perhaps the lesser 
number of teeth in the proportion of four to seven in the upper 
halves of the two jaws of the plaice, to twenty-five to thirty in the 
lower halves, may likewise be accounted for by disuse. From the 
colourless state of the ventral surface of most fishes and of many 
other animals, w^e may reasonably suppose that the absence of 
colour in flat-fish on the side, whether it be the right or left, 
which is undermost, is due to the exclusion of light. But it can- 
not be supposed that the peculiar speckled appearance of the upper 
side of the sole, so like the sandy bed of the sea, or the power in 
some species, as recently shown by Pouchet, of changing their 
colour in accordance with the surrounding surface, or the presence 
of bony tubercles on the upper side of the turbot, are due to the 
action of the light. Here natural selection has probably come into 
play, as well as in adapting the general shape of the body of these 
fishes, and many other peculiarities, to their habits of life. We 
should keep in mind, as I have before insisted, that the inherited 
effects of the increased use of parts, and perhaps of their disuse, will 
be strengthened by natural selection. For all spontaneous varia- 
tions in the right direction will thus be preserved ; as will those 
individuals which inherit in the highest degree the effects of the 
increased and beneficial use of any part. How much to attribute in 
each particular case to the effects of use, and how much to natural 
selection, it seems impossible to decide. 

I may give auDther instance of a structure which apparently owes 
its origin exclusively to use or habit. The extremity of the tail in 
some American monkeys has been converted into a wonderfully 
perfect prehensile organ, and serves as a fifth hand. A reviewer 
who agrees with Mr. Mivart in every detail, remarks on this struc- 
ture : " It is impossible to believe that in any number of ages the 
first slight incipient tendency to grasp could preserve the lives of 


the individuals possessing it, or favour their chance of having and 
of rearing offspring." But there is no necessity for any such belief. 
Habit, and this almost implies that some benefit great or small is 
thus derived, would in all probability suffice for the work. Brehm 
saw the young of an African monkey (Cercopithecus) clinging to 
the under surface of their mother by their hands, and at the same 
lime they hooked their little tails round that of their mother. 
Professor Henslow kept in confinement some harvest mice (Mua 
messorius) which do not possess a structurally prehensile tail ; but 
he frequently observed that they curled their tails round the 
branches of a bush placed in the cage, and thus aided themselves 
in cHmbing. I have received an analogous account from Dr. 
Giinther, who has seen a mouse thus suspend itself. If the harvest 
mouse had been more strictly arboreal, it would perhaps have had 
its tail rendered structurally prehensile, as is the case with some 
members of the same order. Why Cercopithecus, considering its 
habits whilst young, has not become thus provided, it would be 
difficult to say. It is, however, possible that the long tail of this 
monkey may be of more service to it as a balancing organ in 
making its prodigious leaps, than as a prehensile organ. 

The mammary glands are common to the whole class of mam- 
mals, and are indispensable for their existence ; they must, there- 
fore, have been developed at an extremely remote period, and we 
can know nothing positively about their manner of development. 
Mr. Mivart asks : " Is it conceivable that the young of any animal 
was ever saved from destruction by accidentally sucking a drop of 
scarcely nutritious fluid from an accidentally hypertrophied cuta- 
neous gland of its mother ? And even if one was so, what chance 
was there of the perpetuation of such a variation ? " But the case 
is not here put fairly. It is admitted by most evolutionists that 
mammals are descended from a marsupial form ; and if so, the 
mammary glands will have been at first developed within the mar- 
supial sack. In the case of the fish (Hippocampus) the eggs are 
hatched, and the young are reared for a time, within a sack of thia 
nature ; and an American naturalist, Mr. Lockwood, believes from 
what he has seen of the development of the young, that they are 
nourished by a secretion from the cutaneous glands of the sack. 
Now with the early progenitors of mammals, almost before they 
deserved to be thus designated, is it not at least possible that the 
young might have been similarly nourished ? And in this case, 
the individuals which secreted a fluid, in some degree or manner 
the most nutritious, so as to partake of the nature of milk, would 


in the long run have reared a larger numher cf well-nourished off- 
spring, than would the individuals which secreted a poorer fluid ; 
and thus the cutaneous glands, which are the homologues of the 
mammary glands, would have been improved or rendered more 
effective. It accords with the widely extended principle of speciali- 
sation, that the glands over a certain space of the sack should have 
become more highly developed than the remainder ; and they would 
then have formed a breast, but at first without a nipple, as we see in 
the Ornithorhyncus, at the base of the mammalian series. Through 
what agency the glands over a certain space became more highly 
specialised than the others, I will not pretend to decide, whether 
in part through compensation of growth, the effects of use, or of 
natural selection. 

The development of the mammary glands would have been of no 
service, and could not have been effected through natural selection, 
unless the young at the same time were able to partake of the 
secretion. There is no greater difficulty in understanding how 
young mammals have instinctively learnt to suck the breast, than 
in understanding how unhatched chickens have learnt to break the 
egg-shell by tapping against it with their specially adapted beaks ; 
or how a few hours after leaving the shell they have learnt to pick 
up grains of food. In such cases the most probable solution seems 
to be, that the habit was at first acquired by practice at a more 
advanced age, and afterwards transmitted to the offspring at an 
earlier age. But the young kangaroo is said not to suck, only to 
cling to the nipple of its mother, who has the power of injecting 
milk into the mouth of her helpless, half-formed offspring. On this 
head Mr. Mivart remarks : " Did no special provision exist, the 
young one must infallibly be choked by the intrusion of the milk 
into the windpipe. But there is a special provision. The larynx 
is so elongated that it rises up into the posterior end of the nasal 
passage, and is thus enabled to give free entrance to the air for the 
lungs, while the milk passes harmlessly on each side of this elon- 
gated larynx, and so safely attains the gullet behind it." Mr. Mivart 
then asks how did natural selection remove in the adult kangaroo 
(and in most other mammals, on the assumption that they are 
descended from a marsupial form), " this at least perfectly innocent 
and harmless structure ? " It may be suggested in answer that the 
voice, which is certainly of high importance to many animals, could 
hardly have been used with full force as long as the larynx entered 
the nasal passage ; and Professor Flower has suggested to me that 
this structure would have greatly interfered with an animal swallow- 
ing; solid food. 


We will now turn for a short space to the lower divisions of tiie 
animal kingdom. The Echinodermata (star-fishes, sea-urchins, &c.) 
are furnished with remarkable organs, called pediccllaria3, which 
consist, when well developed, of a tridactyle forceps — that is, of one 
formed of three serrated arms, neatly fitting together and placed on 
the summit of a flexible stem, moved by muscles. These forceps 
can seize firmly hold of any object ; and Alexander Agassiz has 
seen an Echinus or sea-urchin rapidly passing particles of excrement 
from forceps to forceps down certain lines of its body, in order that 
its shell should not be fouled. But there is no doubt that besides 
removing dirt of all kinds, they subserve other functions ; and one 
of these apparently is defence. 

With respect to these organs, Mr. Mivart, as on so many pre- 
vious occasions, asks : " What would be the utility of the first 
rudimentary heginnings of such structures, and how could such 
incipient buddings have ever preserved the life of a single Echinus ? '' 
He adds, " not even the sudden development of the snapping action 
could have been beneficial without the freely moveable stalk, nor 
could the latter have been efficient without the snapping jaws, yet 
no minute merely indefinite variations could simultaneously evolve 
these complex co-ordinations of structure ; to deny this seems to do 
no less than to af&rm a startling paradox." Paradoxical as this 
may appear to Mr. Mivart, tridactyle forcepses, immovably fixed 
at the base, but capable of a snapping action, certainly exist on 
some star-fishes ; and this is intelligible if they serve, at least in 
part, as a means of defence. Mr. Agassiz, to whose great kindness 
I am indebted for much information on the subject, informs me 
that there are other star-fishes, in which one of the three arms of 
the forceps is reduced to a support for the other two ; and again, 
other genera in which the third arm is completely lost. In Echino- 
neus, the shell is described by M. Perrier as bearing two kinds of 
pedicellarias, one resembling those of Echinus, and the other those 
of Spatangus; and such cases are always interesting as affording 
the .means of apparently sudden transitions, through the abortion of 
one of the two states of an organ. 

With respect to the steps by which these curious organs have 
been evolved, Mr. Agassiz infers from his own researches and those 
of Miiller, that both in star-fishes and sea-urchins the pedicellarise 
must imdoubtedly be looked at as modified spines. This may be 
inferred from their manner of development in the individual, as 
well as from a long and perfect series of gradations in different 
species and genera, from simple granules to ordinary spines, to 
perfect tridactyle pedi4}ellari£e. The gradation extends evei ir 


the manner in which ordinary spines and the pedicellariaa with 
their supporting calcareous rods are articulated to the shell. Li 
certain genera of star-fishes, "the very combinations needed to 
show that the pedicellaria3 are only modified branching spines" 
may be found. Thus we have fixed spines, with three equi-distant, 
serrated, moveable branches, articulated to near their bases ; and 
higher up, on the same spine, three other moveable branches. 
Xow when the latter arise from the summit of a spine they form 
in fact a rude tridactyle pedicellaria, and such may be seen on the 
same spine together with the three lower branches. In this case 
the identity in nature between the arms of the pedicellarise and the 
moveable branches of a spine, is unmistakeable. It is generally 
admitted that the ordinary spines serve as a protection ; and if so, 
there can be no reason to doubt that those furnished with serrated 
and moveable branches likewise serve for the same purpose ; and 
they would thus serve still more effectively as soon as by meeting 
together they acted as a prehensile or snapping apparatus. Thus 
every gradation, from an ordinary fixed spine to a fixed pedicellaria, 
would be of service. 

In certain genera of star-fishes these organs, instead of being 
fixed or borne on an immovable support, are placed on the summit 
of a flexible and muscular, though short, stem ; and in this case 
they probably subserve some additional function besides defence. 
In the sea-urchins the steps can be followed by which a fixed spine 
becomes articulated to the shell, and is thus rendered moveable. 
I wish I had space here to give a fuller abstract of Mr. Agassiz's 
interesting observations on the development of the pedicellarias. 
All possible gradations, as he adds, may likewise be found between 
the pedicellarias of the star-fishes and the hooks of the Ophiuriaus, 
another group of the Echinodermata ; and again between the pedi- 
cellarise of sea-urchins and the anchors of the Holothurias, also 
belonging to the same great class. 

Certain compound animals, or zoophytes as they have been 
termed, namely the Polyzoa, are provided with curious organs 
called avicularia. These difi'er much in structure in the different 
species. In their most perfect condition, they curiously resemble the 
head and beak of a vulture in miniature, seated on a neck and cap- 
able of movement, as is likewise the lower jaw or mandible. In one 
species observed by me all the avicularia on the same branch often 
moved simultaneously backwards and foi^ards, with the lower 
jaw widely open, through an angle of about 90°. in the course of 
five seconds; and then movement caused the whole polyzoary to 


tremble. When the jaws are touched with a needle they seize it 
m) firmly that the branch can thus be shaken. 

Mr. Mivart adduces this case, chiefly on account of the supposed 
difficulty of organs, namely the avicularia of the Polyzoa and the 
pedicellarise of the Echinodermata, which he considers as " essen- 
tially similar," having been developed through natural selection in 
widely distinct divisions of the animal kingdom. But, as far as 
structure is concerned, I can see no similarity between tridactyle 
pedicellariae and avicularia. The latter resemble somewhat more 
closely the chela3 or pincers of Crustaceans ; and Mr. Mivart might 
have adduced with equal appropriateness this resemblance as a 
special difficulty ; or even their resemblance to the head and beak 
of a bird. U'he avicularia are believed by Mr. Busk, Dr. Smitt, and 
Dr. Nitsche — naturalists who have carefully studied this group — to 
be homologous with the zooids and their cells which compose the 
zoophyte ; the moveable lip or lid of the cell corresponding with the 
lower and moveable mandible of the avicularium. Mr, Busk, how- 
ever, does not know of any gradations now existing between a zooid 
and an avicularium. It is therefore impossible to conjecture by 
what serviceable gradations the one could have been converted into 
the other : but it by no means follows from this that such grada- 
tions have not existed. 

As the chelse of Crustaceans resemble in some degree the avicu- 
laria of Polyzoa, both serving as pincers, it may be worth while to 
show that with the former a long series of serviceable gradations 
still exists. In the first and simplest stage, the terminal segment 
of z. limb shuts down either on the square summit of the broad 
penultimate segment, or against one whole side ; and is thus enabled 
to catch hold of an object ; but the limb still serves as an organ 
of locomotion. We next find one corner of the broad penultimate 
segment slightly prominent, sometimes furnished with irregular 
teeth ; and against these the terminal segment shuts down. By an 
increase in the size of this projection, with its shape, as well as that 
of the terminal segment, slightly modified and improved, the pincers 
are rendered more and more perfect, until we have at last an instru- 
ment as efficient as the chela) of a lobster ; and all these gradations 
can be actually traced. 

Besides the avicularia, the Polyzoa possess curious organs called 
viljacula. These generally consist of long bristles, capable of 
movement and easily excited. In one species examined by me 
the vibracula were slightly curved and serrated along the outer 
margin ; and all of them on the same polyzoary often moved simul- 
taneously ; so that, acting like long oars, they swept a branch rapidly 


'i ' ' <j 

Staif r--' ^, . ^ 


across the object-glass of my microscope. When a branch was 
placed on its face, the vihracula became entangled, and they made 
violent efforts to free themselves. They are supposed to serve as 
a defence, and may be seen, as Mr. Busk remarks,* "to sweep 
slowly and carefully over the surface of the polyzoary, removing 
what might be noxious to the delicate inhabitants of the cells when 
their tentacula are protruded." The avicularia, like the vibracula, 
probably serve for defence, but they also catch and kill small living 
animals, which it is believed are afterwards swept by the currents 
within reach of the tentacula of the zooids. Some species are 
provided with avicularia and vibracula ; some With avicularia alone, 
and a few with vibracula alone. 

It is not easy to imagine two objects more widely different in 
appearance than a bristle or vibraculum, and an avicularium like 
the head of a bird ; yet they are almost certainly homologous and 
have been developed from the same common source, namely a zooid 
with its cell. Hence we can understand how it is that these 
organs graduate in some cases, as I am informed by Mr. Busk, 
into each other. Thus with the avicularia of several species of 
Lepralia, the moveable mandible is so much produced and is so like 
a bristle, that the presence of the upper or fixed beak alone serves 
to determine its avicularian nature. The vibracula may have been 
directly developed from the lips of the cells, without having passed 
through the avicularian stage ; but it seems more probable that 
they have passed through this stage, as during the early stages of 
the transformation, the other parts of the cell with the included 
zooid could hardly have disappeared at once. In many cases the 
vibracula have a grooved support at the base, which seems to repre- 
sent the fixed beak ; though this support in some species is quite 
absent. This view of the development of the vibracula, if trust- 
worthy, is interesting ; for supposing that all the species provided 
with avicularia had become extinct, no one with the most vivid 
imagination would ever have thought that the vibracula had originall}'' 
existed as part of an organ, resembling a bird's head or an irregular 
box or hood. It is interesting to see two such widely different 
organs developed from a common origin ; and as the moveable lip 
of the cell serves as a protection to the zooid, there is no difficulty 
in believing that all tbe gradations, by which the lip became con- 
verted first into the lower mandible of an avicularium and then 
into an elongated bristle, likewise served as a protection in different 
ways and under different circumstances. 

lu the vegetable kingdom Mr. Mivart only alludes to t-wo cases, 


namely the stiiicture of the flowers of orchids, and the movements 
of climbing plants. With respect to the former, he says, " the 
explanation of their origin is deemed thoroughly unsatisfactory — 
utterly insuflBcient to explain the incipient, infinitesimal beginnings 
of structures which are of utility only when they are considerably 
developed." As I have fully treated this subject in another work, 
1 will here give only a few details on one alone of the most striking 
peculiarities of the flowers of orchids, namely their pollinia. A 
pollinium when highly developed consists of a mass of pollen-grains, 
affixed to an elastic foot-stalk or caudicle, and this to a little mass 
of extremely viscid matter. The pollinia are by this means trans- 
ported by insects from one flower to the stigma of another. In 
some orchids there is no caudicle to the pollen-masses, and the 
grains are merely tied together by fine threads ; but as these are 
not confined to orchids, they need not here be considered ; yet I 
may mention that at the base of the orchidaceous series, in Cypri- 
pedium, we can see how the threads were probably first developed. 
In other orchids the threads cohere at one end of the pollen-masses ; 
and this forms the first or nascent trace of a caudicle. That this 
is the origin of the caudicle, even when of considerable length and 
highly developed, we have good evidence in the aborted pollen- 
grains which can sometimes be detected embedded within the 
central and solid parts. 

With respect to the second chief peculiarity, namely the little 
mass of viscid matter attached to the end of the caudicle, a long 
series of gradations can be specified, each of plain service to the 
plant. In most flowers belonging to other orders the stigma se- 
cretes a little viscid matter. Now in certain orchids similar viscid 
matter is secreted, but in much larger quantities by one alone of 
the three stigmas ; and this stigma, perhaps in consequence of the 
copious secretion, is rendered sterile. When an insect visits a flower 
of this kind, it rubs off some of the viscid matter an(i thus at the 
same time drags away some of the pollen -grains. From this simple 
condition, which differs but little from that of a multitude of 
common flowers, there are endless gradations, — to species in which 
the pollen-mass terminates in a very short, free caudicle, — to others 
in which the caudicle becomes firmly attached to the viscid matter, 
with the sterile stigma itself much modified. In this latter case 
we have a pollinium in its most highly developed and perfect con- 
dition. He who will carefully examine the flowers of ordiids for 
himself will not deny the existence of the above series of gradations 
— from a mass of pollen-grains merely tied together by threads, 
with the stigma differing but little from that v^f .aiv ordinary flower, 



to a highly complex poUinium, admirably adapted for transportaJ 
by insects ; nor will he deny that all the gradations in the several 
species are admirably adapted in relation to the general * structure 
of each flower for its fertilisation by different insects. In this, and 
in almost every other case, the enquiry may be pushed further 
backwards ; and it may be asked how did the stigma of an ordinary 
flower become viscid, but as we do not know the full history of any 
one group of beings, it is as useless to ask, as it is hopeless to 
attempt answering, such questions. 

We will now turn to climbing plants. These can be arranged in 
a long series, from those which simply twine round a support, to 
those which I have called leaf-climbers, and to those provided with 
tendrils. In these two latter classes the stems have generally, but 
not always, lost the power of twining, though they retain the power 
of revolving, which the tendrils likewise possess. The gradations 
from leaf-climbers to tendril-bearers are wonderfully close, and 
certain plants may be indifferently placed in either class. But in 
ascending the series from simple twiners to leaf-climbers, an impor- 
tant quality is added, namely sensitiveness to a touch, by which 
means the foot-stalks of the leaves or flowers, or these modified 
and converted into tendrils, are excited to bend round and clasp 
the touching object. He who will read my memoir on these plants 
will, I think, admit that all the many gradations in function and 
structure between simple twiners and tendril-bearers are in each 
case beneficial in a high degree to the species. For instance, it 
is clearly a great advantage to a twining plant to become a leaf- 
climber ; and it is probable that every twiner which possessed 
leaves with long foot-stalks would have been developed into a leaf- 
climber, if the foot-stalks had possessed in any slight degree the 
requisite sensitiveness to a touch. 

As twining is tlie simplest means of ascending a support, and 
forms the basis of our series, it may naturally be asked how did 
plants acquire this power in an incipient degree, afterwards to be 
improved and increased through natural selection. The power of 
twining depends, firstly, on the stems whilst young being extremely 
flexible (but this is a character common to many plants which are 
not climbers) ; and, secondly, on their continually bending to all 
points of the compass, one after the other in succession, in the same 
order. By this movement the stems are inclined to all sides, and 
are made to move round and round. As soon as the lowei- part A 
of a stem strikes against any object and is stopped, the upper part 
still goes on bending and revolving, and thus necessarily twines 
round and up the support. The revolving movement ;eases after 


the early growth of each shoot. As in many widely separated 
families of plants, single species and single genera possess the powei 
of revolving, and have thus become twiners, they must have 
independently acquired it, and cannot have inherited it from a 
common progenitor. Hence I was led to predict that some slight 
tendency to a movement of this kind would be found to be far from 
uncommon with plants which did not climb ; and that this had 
afforded the basis for natural selection to work on and improve. 
When I made this prediction, I knew of only one imperfect case, 
namely of the young flower-peduncles of a Maurandia which 
revolved slightly and irregularly, like the stems of twining plants, 
but without making any use of this habit. Soon afterwards Fritz 
MUller discovered that the young stems of an Alisma and of a 
Linum, — plants which do not climb and are widely separated in 
the natural system, — revolved plainly, though irregularly ; and he 
states that he has reason to suspect that this occurs with some other 
plants. These slight movements appear to be of no service to the 
plants in question ; anyhow, they are not of the least use in the way 
of climbing, w^hich is the point that concerns us. Nevertheless we can 
see that if the stems of these plants had been flexible, and if under 
the conditions to which they are exposed it had profited them to as- 
cend to a height, then the habit of slightly and irregularly revolving 
might have been increased and utilised through natural selection, 
until they had become converted into well-developed twining species. 
With respect to the sensitiveness of the foot-stalks of the leaves 
and flowers, and of tendrils, nearly the same remarks are applicable 
as in the case of the revolving movements of twining plants. As 
a vast number of species, belonging to widely distinct groups, are 
endowed with, this kind of sensitiveness, it ought to be found in a 
nascent condition in many plants which have not become climbers. 
This is the case : I observed that the young flower-peduncles of 
the above Maurandia curved themselves a little towards the side 
which was touched. Morren found in several species of Oxalis that 
the leaves and their foot-stalks moved, especially after exposure 
to a hot sun, when they were gently and repeatedly touched, :>r 
when the plant was shaken. I repeated these observations on some 
other species of Oxalis with the same result ; in some of them the 
movement was distinct, but was best seen in the young leaves ; in 
others it was extremely slight. It is a more important fact that 
according to the high authority of Hofmeister, the young shoots and 
leaves of all plants move after being shaken ; and with climbing 
plants it is, as we know, only during the early stage? z( growth that 
the foot-stalks and tendrils are sensitive. 


It is scarcely possible that the above slight movements, due to 
a touch or shake, in the young aud growing organs of plants, can 
be of any functional importance to them. But plants possess, in 
obedience to various stimuli, powers of movement, which are of 
manifest importance to them ; for instance, towards and more rarely 
from the light, — in opposition to, and more rarely in the direction 
of, the attraction of gravity. When the nerves and muscles of an 
animal are excited by galvanism or by the absorption of strychnine, 
the consequent movements may be called an incidental result, for 
the nerves and muscles have not been rendered specially sensitive to 
these stimuli. So with plants it appears that, from having the 
power of movement in obedience to certain stimuli, they are excited 
in an incidental manner by a touch, or by being shaken. Hence 
there is no great difiiculty in admitting that in the case of leaf- 
climbers and tendril-bearers, it is this tendency which has been 
taken advantage of and increased through natural selection. It is, 
however, probable, from reasons which I have assigned in my 
memoir, that this will have occurred only with plants which had 
already acquired the power of revolving, and had thus become 

I have already endeavoured to explain how plants became twiners, 
namely, by the increase of a tendency to slight and irregular 
revolving movements, which were at first of no use to them ; this 
movement, as well as that due to a touch or shake, being the inci- 
dental result of the power of moving, gained for other and bene- 
ficial purposes. Whether, during the gradual development of 
climbing plants, natural selection has been aided by the inherited 
effects of use, 1 will not pretend to decide; but we know that 
certain periodical movements, for instance the so-called sleep of 
plants, are governed by habit. 

I have now considered enough, perhaps more than enough, of the 
cases, selected with care by a skilful naturalist, to prove that natural 
selection is incompetent to account for the incipient stages of useful 
structures; and I have shown, as I hope, that there is no great 
difficulty on this head. A good opportunity has thus been afforded 
for enlarging a little on gradations of structure, often associated 
with changed functions, — an important subject, which was not 
treated at sufficient length in the former editions of this work. I 
will now briefly recapitulate the foregoing cases. 

With the giraffe, the contmued preservation of the individuals ot 
some extinct high-reaching ruminant, which had the longest necks, 
legs, &c., and could browse a little above the average height, and 


tlie continued destruction of those which could not browse so hicrh 
would have sufficed for the production of this remarkable quad- 
ruped ; but the prolonged use of all the parts together with inherit- 
ance will have aided in an important manner in their co-ordination. 
With the many insects which imitate various objects, there is no 
improbability in the belief that an accidental resemblance to some 
common object was in each case the foundation for the work of 
natural selection, since perfected through the occasional preservation 
of slight variations which made the resemblance at all closer ; and 
this will have been carried on as long as the insect continued to 
vary, and as long as a more and more perfect resemblance led to its 
escape from sharp-sighted enemies. In certain species of whales 
there is a tendency to the formation of irregular little points of horn 
on the palate ; and it seems to be quite within the scope of natural 
selection to preserve all favourable variations, until the points were 
converted first into lamellated knobs or teeth, like these on the 
"beak of a goose, — then into short lamellae, like those of the domestic 
ducks, — and then into lamellse, as perfect as those of the shoveller- 
duck, — and finally into the gigantic plates of baleen, as in the mouth 
of the Greenland whale. In the family of the ducks, the lamellae 
are first used as teeth, then partly as teeth and partly as a sifting 
apparatus, and at last almost exclusively for this latter purpose. 

With such structures as the above lamellae of horn or whale- 
bone, habit or use can have done little or nothing, as far as we 
can judge, towards their development. On the other hand, the 
transportal of the lower eye of a flat-fish to the upper side of 
the head, and the formation of a prehensile tail, may be attributed 
almost wholly to continued use, together with inheritance. With 
respect to the mammae of the higher animals, the most probable 
conjecture is that primordially the cutaneous glands over the whole 
surface of a marsupial sack secreted a nutritious fluid ; and that 
these glands were improved in function through natural selection, 
and concentrated into a confined area, in which case they would 
have formed a mamma. There is no more difficulty in under- 
standing how the branched spines of some ancient Echinoderm, 
which served as a defence, became developed through natural selec- 
tion into tridactyle pedicellari^e, than in understanding the develop- 
ment of the pincers of crustaceans, through slight, serviceable modi- 
fications in the ultimate and penultimate segments of a limb, 
which was at first used solely for locomotion. In the avicularia 
and vibracula of the Polyzoa we have organs w idely different in 
appearance developed trom the same source ; and w ith the vibracula 
y^ii can understand how the successive gradations might have been 


of service. With the pollinia of orchids, the threads which originally 
served to tie together the pollen-grains, can be traced cohering into 
caudicles ; and the steps can likewise be followed by which viscid 
matter, such as that secreted by the stigmas of ordinary flowers, and 
Btill subserving nearly but not quite the same purpose, became 
attached to the free ends of the caudicles; — all these gradations 
being of manifest benefit to the plants in question. With respect 
to climbing plants, I need not repeat what has been so lately said. 

It has often been asked, if natural selection be so potent, why has 
not this or that structure been gained by certain species, to which it 
would apparently have been advantageous ? But it is unreasonable to 
expect a precise answer to such questions, considering our ignorance 
of the past history of each species, and of the conditions which at 
the present day determine its numbers and range. In most cases 
only general reasons, but in some few cases special reasons, can be 
assigned. Thus to adapt a species to new habits of life, many co- 
ordinated modifications are almost indispensable, and it may often 
have happened that the requisite parts did not vary in the right 
manner or to the right degree. Many species must have been 
prevented from increasing in numbers through destructive agencies, 
which stood in no relation to certain structures, which we imagine 
would have been gained through natural selection from appearing 
to us advantageous to the species. In this case, as the struggle 
for life did not depend on such structures, they could not have 
been acquired through natural selection. In many cases complex 
and long-enduring conditions, often of a peculiar nature, are neces- 
sary for the development of a structure ; and the requisite con- 
ditions may seldom have concurred. The belief that any given 
structure, which we think, often erroneously, would have been 
beneficial to a species, would have been gained under all circum- 
stances through natural selection, is opposed to w^hat we can under- 
stand of its manner of action. Mr. Mivart docs not deny that 
natural selection has effected something ; but he considers it as 
*' demonstrably insufficient " to account for the phenomena which I 
explain by its agency. His chief arguments have now been con- 
sidered, and the others will hereafter be considered. They seem to 
me to partake little of the character of demonstration, and to have 
little weight in comparison with those in favour of the power of 
natural selection, aided by the other agencies often specified. I am 
bound to add, that some of the facts and arguments here used by 
me, have been advanced for the same purpose in an able article 
lately published in the ' Medico-Chirurgical Review.' 


At the present day almost all naturalists admit evolution under 
some form. Mr. Mivart believes that species change through "an 
internal force or tendency," about which it is not pretended that 
anything is known. That species have a capacity for change will 
be admitted by all evolutionists ; but there is no need, as it seems 
to me, to invoke any internal force beyond the tendency to ordi- 
nary variability, "which through the aid of selection by man has 
given rise to many well-adapted domestic races, and w^iich through 
the aid of natural selection would equally well give rise by gradu- 
ated steps to natural races or species. The final result will gene- 
rally have been, as already explained, an advance, but in some few 
cases a retrogression, in organisation. 

Mr. Mivart is further inclined to believe, and soni-e naturalists 
agree with him, that new species manifest themselves " with sud- 
denness and by modifications appearing at once." For instance, 
he supposes that the differences between the extinct three-toed 
Hipparion and the horse arose suddenly. He thinks it difficult to 
believe that the wing of a bird " was developed in any other way 
than by a comparatively sudden mt)dification of a marked and 
important kind ; " and apparently he would extend the same view- 
to the wings of bats and pterodactyles. This conclusion, which 
implies great breaks or discontinuity in the series, appears to me 
improbable in the highest degree. 

Every one who believes in slow and gradual evolution, will of 
course admit that specific changes may have been as abrupt and as 
great as any single variation which we meet with under nature, 
or even under domestication. But as species are more variable 
when domesticated or cultivated than under their natural con- 
ditions, it is not probable that such great and abrupt variations 
have often occurred under nature, as are known occasionally to 
arise under domestication. Of these latter variations several may 
be attributed to reversion ; and the characters which thus reappear 
were, it is probable, in many cases at first gained in a gradual 
manner. A still greater number must be called monstrosities, such 
as six-fin.gered men, porcupine men, Ancon slieep, Niata cattle, &c. ; 
and as they are widely different in character from natural species, 
they throw very little light on our subject. Excluding such cases 
of abrupt variations, the few which remain would at best constitute, 
if found in a state of nature, doubtful species, closely related to 
their parental types. 

My reasons for doubting whether natural species have changed 
as abruptly as have occasionally domestic races, and for entirely 
disbelieving that they have changed in the wonderful mannej 


indicated by Mr. Mivart, are as follows. According to our expe- 
rience, abrupt and strongly marked variations occur in our domesti- 
cated productions, singly and at rather long intervals of time. If 
such occurred under nature, they would be liable, as formerly 
explained, to be lost by accidental causes of destruction and by 
Bubsequent inter-crossing ; and so it is known to be under domesti- 
cation, unless abrupt variations of this kind are specially preserved 
and separated by the care of man. Hence in order that a new 
species should suddenly appear in the manner supposed by Mr. 
Mivart, it is almost necessary to believe, in opposition to all ana- 
logy, that several wonderfully changed individuals appeared simul- 
taneously within the same district. This difficulty, as in the case of 
unconscious selection by man, is avoided on the theory of gradual 
evolution, through the preservation of a large number of individuals, 
which varied more or less in any favourable direction, and of the 
destruction of a large number which varied in an opposite manner. 

That many species have been evolved in an extremely gradual 
manner, there can hardly be a doubt. The species and even the 
genera of many large natural families are so closely allied together, 
that it is difficult to distinguish not a few of them. On every con- 
tinent in proceeding from north to south, from lowland to upland, 
&c., we meet with a host of closely related or representative species ; 
as we likewise do on certain distinct continents, which we have 
reason to believe were formerly connected. But in making these 
and the following remarks, I am compelled to allude to subjects 
hereafter to be discussed. Look at the many outlying islands round 
a continent, and see how many of their inhabitants can be raised 
only to the rank of doubtful species. So it is if we look to past 
times, and compare the species which have just passed away with 
those still living within the same areas ; or if we compare the fossil 
species embedded in the sub-stages of the same geological formation. 
It is indeed manifest that multitudes of species are related in the 
closest manner to other species that still exist, or have lately 
existed ; and it will hardly be maintained that such s[iecies have 
been developed in an abrupt or sudden manner. Nor should it be 
forgotten, when we look to the special parts of allied species, instead 
of to distinct species, that numerous and wonderfully fine grada- 
tions can be traced, connecting together widely different structures. 

Many large groups of facts are intelligible only on the principle 
that species have been evolved by very small steps. For instance, 
the fact that the species included in the larger genera are more closely 
related to each other, and present a greater number of varieties 
tha;' do the species in the smaller genera. The former are al»o 


grouped in little clusters, like varieties round species; and they 
present other analogies with varieties, as was shown in our second 
chapter. On this same principle we can understand how it is that 
specific characters are more variable than generic characters ; and 
how the parts which are developed in an extraordinary degree 
or manner are more variable than other parts of the same species. 
Many analogous facts, all pointing in the same direction, could be 

Although very many species have almost certainly been pro- 
duced by steps not greater than those separating fine varieties ; yet 
It may be maintained that some have been developed in a different 
and abrupt manner. Such an admission, however, ought not to be 
made without strong evidence being assigned. The vague and in 
some respects false analogies, as they have been shown to be by 
Mr. Chauncey Wright, which have been advanced in favour of this 
view, such as the sudden crystallisation of inorganic substances, or 
the falling of a facetted spheroid from one facet to another, hardly 
deserve consideration. One class of facts, however, namely, the 
sudden appearance of new and distinct forms of life in our geological 
formations supports at first sight the belief in abrupt development. 
But the value of this evidence depends entirely on the perfection ol 
the geological record, in relation to periods remote in the history 
of the world. If the record is as fragmentary as many geologists 
strenuously assert, there is nothing strange in new forms appear- 
ing as if suddenly developed. 

Unless we admit transformations as prodigious as those advocated 
by Mr. Mivart, such as the sudden development of the wings of 
birds or bats, or the sudden conversion of a Hipparion into a norse, 
hardly any light is thrown by the belief in abrupt modifications on 
the deficiency of connecting links in our geological formations. But 
against the belief in such abrupt changes, embryology enters a strong 
protest. It is notorious that the wings of birds and bats, and the legs 
of horses or other quadrupeds, are undistinguishable at an early em- 
bryonic period, and that they become ditlerentiated by insensibly 
fine steps. Embryological resemblances of all kinds can be ac- 
counted for, as we shall hereafter see, by the progenitors of our 
existing species having varied after early youth, and having trans- 
mitted their newly acquired characters to their offspring, at a 
corresponding age. The embryo is thus left almost unaffected, and 
serves as a record of the past condition of the species. Hence it 
is that existing species during the early stages of their develop- 
ment so often resemble ancient and extinct forms belonging to the 
same class. On this view of the meaning of embryological rcsem- 


blances, and indeed on any view, it is incredible that an animal 
should have undergone such momentous and abrupt transforma- 
tions, as those above indicated ; and yet should not bear even a 
trace in its embryonic condition of any sudden modification ; every 
detail in its structure being developed by insensibly fine steps. 

He who believes that some ancient form was transformed sud- 
denly through an internal force or tendency into, for instance, one 
furnished with wings, will be almost compelled to assume, in oppo- 
"sition to all analogy, that many individuals varied simultaneously. 
It cannot be denied that such abrupt and great changes of struc- 
ture are widely different from those which most species apparently 
have undergone. He will further be compelled to believe that 
many structures beautifully adapted to all the other parts of the 
same creature and to the surrounding conditions, have been sud- 
denly produced ; and of such complex and wonderful co-adapta- 
tions, he will not be able to assign a shadow of an explanation. 
He will be forced to admit that these great and sudden transfor- 
mations have left no trace of their action on the embryo. To 
admit all this ib, as it seems to me, to enter into the realma of 
mlKicle, and to leave those of Science. 

Cbmv. VIII.] INSTINCT. 205 



Instincts comparable with habits, but different in their origin — Instincts 
graduated — Aphides and ants — Instincts variable — Domestic ia 
stincts, their origin — Natural instincts of the cuckoo, molothrus, 
ostrich, and parasitic bees — Slave-making ants — Hive-bee, its cell- 
making instinct — Changes of instinct and structure not necessarily 
simultaneous — Difficulties of the theory of the Natural Selection of 
instincts — Neuter or sterile insects — Summary. 

Many instincts are so wonderful that their development will pro- 
bably appear to the reader a difficulty sufficient to overthrow m^ 
whole theory. I may here premise, that I have nothing to do with 
the origin of the mental powers, any more than I have with that of 
life itself. We are concerned only with the diversities of instinct 
and of the other mental faculties in animals of the same class. 

I will not attempt any definition of instinct. It would be easy to . , , 
show that several distinct mental actions are commonly embraced Z^^-'"'^-'^ 
by this term ; but every one, understands what is meant, when it is ^ 
said that instinct impels the cuckoo to migrate and to lay her eggs 
in other birds' nests. An action, which we ourselves require expe- 
rience to enable us to perform, when performed by an animal, more 
especially by a very young one, without experience, and when per- 
formed by many individuals in the same way, without their knowing 
for what purpose it is performed, is usually said to be instinctive. 
But I could show that none of these characters are universal. A 
little dose of judgment or reason, as Pierre Huber expresses it, oftep 
comes into play, even with animals low in the scale of nature. 

Frederick Cuvier and several of the older metaphysicians have 
compared instinct with habit. This comparison gives, I think, an 
accurate notion of the frame of mind under which an instinctive 
action is performed, but not necessarily of its origin. How uncon- 
sciously many habitual actions are performed, indeed not rarely in 
direct opposition to our conscious will ! yet they may be modi- 
fied by the will or reason. Habits easily become associated with 
other habits, with certain periods of time, and states of the body. 
When once acquired, they often remain constant throughout life. 

200 INSTINCT. [Chap. VIII 

Several other points of resemblance between instincts and habits 
could be pointed out. As in repeating a well-known song, so in 
instincts, one action follows another by a sort of rhythm ; if a person 
be interrupted in a song, or in repeating anything by rote, he is 
generally forced to go back to recover the habitual train of thought. 
so P. Huber found it was with a caterpillar, which makes a very com- 
plicated hanamock ; for if he took a caterpillar which had completed 
its hammock up to, say, the sixth stage of construction, and put it 
into a hammock completed up only to the third stage, the caterpillar 
simply re-performed the fourth, fifth, and sixth stages of construction. 
If, however, a caterpillar were taken out of a hammock made up, 
for instance, to the third stage, and were put into one finished up to 
the sixth stage, so that much of its work was already done for it, 
far from deriving any benefit from this, it was much embarrassed, 
and in order to complete its hammock, seemed forced to start from 
the third stage, where it had left off, and thus tried to complete the 
already finished work. 

If we suppose any habitual action to become inherited — and it 
can be shown that this does sometimes happen — then the resem- 
blance between what originally was a habit and an instinct becomes 
so close as not to be distinguished. If Mozart, instead of playing 
the pianoforte at three years old with wonderfully little practice, 
had played a tune with no practice at all, he might truly be said 
to have done so instinctively. But it would be a serious error to 
suppose that the greater number of instincts have been acquired by 
habit in one generation, and then transmitted by inheritance to 
succeeding generations. It can be clearly shown that the most 
wonderful instincts with which we are acquainted, namely, those 
of the hive-bee and of many ants, could not possibly have been 
acquired by habit. 

It will be universally admitted that instincts are as important as 
corporeal structures for the welfare of each species, under its present 
"conditions of life. Under changed conditions of life, it is at least 
possible that slight modifications of instinct might be profitable to 
a species ; and if it can be shown that instincts do vary ever so 
little, then I can see no difiiculty in natural selection preservings 
and continually accumulating variations of instinct to any extent 
that was profitable. It is thus, as I believe, that all the most 
complex and wonderful instincts have originated. As modifications 
of corporeal structure arise from, and are increased by, use or habit, 
and are diminished or lost by disuse, so I do not doubt it has been 
with instincts. But I believe that the effects of habit are in many of subordinate importance to the effects of the natural selection 

Chap. VIII.] LVSTINCT. 207 

of wLat may he called ppontancons variations of instincts ; — that 
is of variations produced by the same unknown causes which pro- 
duce slight deviations of bodily structure. 

No complex instinct can possibly be produced through natural 
selection, except by the slow and gradual accumulation of numerous 
slight, yet profitable, variations. Hence, as in the case of corporeal 
structures, we ought to find in nature, not the actual transitional 
gradations by which each complex instinct has been acquired — for 
these could be found only in the lineal ancestors of each species — 
but we ought to find in the collateral lines of descent some evidence 
of such gradations ; or we ought at least to be able to show that 
gradations of some kind are possible ; and this we certainly can do. 
I have been surprised to find, making allowance for the instincts of 
animals having been but little observed except in Europe and North 
America, and for no instinct being known amongst extinct species, 
how very generally gradations, leading to the most complex instincts, 
can be discovered. Changes of instinct may sometimes be facilitated 
by the same species having different instincts at different periods 
of life, or at different seasons of the year, or when placed under 
different circumstances, &c. ; in which case either the one or the 
other instinct might be preserved by natural selection. And such 
instances of diversity of instinct in the same species can be shown 
to occur in nature. 

Again, as in the case of corporeal structure, and conformably to 
my theory, the instinct of each species is good for itself, but has 
never, as far as we can judge, been produced for the exclusive good 
of others. One of the strongest instances of an animal apparently 
performing an action for the sole good of another, with which I am 
acquainted, is that of aphides voluntarily yielding, as was first 
observed by Iluber, their sweet excretion to ants : that they do so 
voluntarily, the following facts show. I removed all the ants from 
a group of about a dozen aphides on a dock-plant, and prevented 
their attendance during several hours. After this interval, I felt 
sure that the aphides would want to excrete. I watched them for 
some time through a lens, but not one excreted ; I then tickled and 
stroked them with a hair in the same manner, as well as I could, 
as the ants do with their antennae ; but not one excreted. After- 
wards I allowed an ant to visit them, and it immediately seemed, 
by its eager v/ay of running about, to be well aware what a rich 
flock it had discovered ; it then began to play with its antennae on 
the abdomen first of one aphis and then of another ; and each, as 
Boon as it felt the antennae, immediately lifted up its abdomen and 
excreted a limpid drop of sweet juice, which was eagerly devoured 

203 INSTINCT. [Chap. VIU. 

by the ant. Even the quite young aphides lehavcd in this manner, 
showing; that the action was instinctive, and not the result oi 
experience. It is certain, from the observations of Huber, that the 
aphides show no dislike to the ants : if the latter be not present 
they are at last compelled to eject their excretion. But as the 
excretion is extremely viscid, it is no doubt a convenience to the 
aphides to have it removed ; therefore probably they do not excreta 
solely for the good of the ants. Although there is no evidence that 
any animal performs an action for the exclusive good of anothei 
species, yet each tries to take advantage of the instincts of others, 
as each takes advantage of the weaker bodily structure of other 
species. So again certain instincts cannot be considered as abso- 
lutely perfect ; but as details on this and other such points are not 
indispensable, they may be here passed over. 

As some degree of variation in instincts under a state of nature, 
and the inheritance of such variations, are indispensable for the 
action of natural selection, as many instances as possible ought to 
be given ; but want of space prevents me. I can only assert that 
instincts certainly do vary — for instance, the migratory instinct, 
both in extent and direction, and in its total loss. So it is with the 
nests of birds, which vary partly in dependence on the situations 
chosen, and on the nature and temperature of the country inhabited, 
but often from causes wholly unknown to us : Audubon has given 
several remarkable cases of differences in the nests of the same 
species in the northern and southern United States. Why, it has 
been asked, if instinct be variable, has it not granted to the bee 
" the ability to use some other material when wax was deficient " ? 
But what other natural material could bees use ? They will work, 
as I have seen, with wax hardened with vermilion or softened 
with lard. Andrew Knigbt observed that his bees, instead of 
laboriously collecting propolis, used a cement of wax and turpentine, 
with which he had covered decorticated trees. It has lately been 
shown that bees, instead of searching for pollen, will gladly use a 
very different substance, namely oatmeal. Fear of any particular 
enemy is certainly an instinctive quality, as may be seen in nestling 
birds, though it is strengthened by experience, and by the sight of 
fear of the same enemy in other animals. The fear of man is slowly 
acquired, as I have elsewhere shown, by the various animals which 
inhabit desert islands; and we see an instance of this even in 
England, in the greater wildness of all our large birds in comparison 
with our small birds ; for the large birds have been most persecuted 
by man. We may safely attribute the greater wildness of our 
large birds to this cause ; for in uninhabited islands large birds aiv 


rot more fearful than small ; and the magpie, so wary in England, 
is tame in Norway, as is the hooded crow in Egypt. 

That the mental qualities of animals of the same kind, born in a 
state of nature, vary much, could be shown by many facts. Several 
cases could also be adduced of occasional and strange habits in 
wild animals, which, if advantageous to the species, might have 
given rise, through natural selection, to new instincts. But I am 
well aware that these general statements, without the facts in 
detail, will produce but a feeble effect on the reader's mind. I 
can only repeat my assurance, that I do not speak without good 

Inherited Changes of Habit or Instinct in Domesticated 


The possibility, or even probability, of inherited variations of 
instinct in a state of nature will be strengthened by briefly consider- 
ing a few cases under domestication. We shall thus be enabled to 
see the part which habit and the selection of so-called spontaneous 
variations have played in modifying the mental qualities of our 
domestic animals. It is notorious how much domestic animals vary 
in their mental qualities. With cats, for instance, one naturally 
takes to catching rats, and another mice, and these tendencies are 
known to be inherited. One cat, according to Mr. St. John, always 
brought home game-birds, another hares or rabbits, and another 
hunted on marshy ground and almost nightly caught woodcocks or 
snipes. A number of curious and authentic instances could be 
given of various shades of disposition and of taste, and likewise of 
the oddest tricks, associated with certain frames of mind or periods 
of time, being inherited. But let us look to the familiar case of 
the breeds of the dog : it cannot be doubted that young pointers (I 
have myself seen a striking instance) will sometimes point and even 
back other dogs the very first time that they are taken out ; 
retrieving is certainly in some degree inherited by retrievers ; and a 
tendency to run round, instead of at, a flock of sheep, by shepherd - 
dogs. 1 cannot see that these actions, performed without experience 
by the young, and in nearly the same manner by each individual, 
performed with eager delight by each breed, and without the end '*^ 
being known — for the young pointer can no more know that he 
points to aid his master, than the white butterfly knows why she 
lays her eggs on the leaf of the cabbage — I cannot see that these 
actions differ essentially from true instincts. If we were to benold 
one kind of wolf, when young and without any training, as soon as 
U scented its prey, stand motionless like a statue, and then slowly 






crawl forward with a peculiar gait ; and another kind of wolf 
rushing round, instead of at, a herd of deer, and driving them to a 
distant point, we should assuredly call these actions instinctive. 
Domestic instincts, as they may be called, are certainly far less 
nxed than natural instincts ; but they have been acted on by far 
less rigorous selection, and have been transmitted for an incompar- 
ably shorter period, under less fixed conditions of life. 

How strongly these domestic instincts, habits, and dispositions 
are inherited, and how curiously they become mingled, is well 
shown when different breeds of dogs are crossed. Thus it is known 
that a cross with a bull-dog has affected for many generations the 
courage and obstinacy of greyhounds ; and a cross with a greyhound 
has given to a whole family of shepherd-dogs a tendency to hunt 
hares. These domestic instincts, when thus tested by crossing, 
resemble natural instincts, which in a like manner become curiously 
blended together, and for a long period exhibit traces of the instincts 
of either parent : for example, Le Eoy describes a dog, whose^eat- 
grandfather was a wolf, and this dog showed a trace of its wdld 
parentage only in one way, by not coming in a straight line to his 
master, when called. 

Domestic instincts are sometimes spoken of as actions which 
have become inherited solely from long-continued and compulsory 
habit ; but this is not true. No one would ever have thought of 
teaching, or probably could have taught, the tumbler-pigeon to 
tumble, — an action which, as I have witnessed, is performed by 
young birds, that have never seen a pigeon tumble. We may 
believe that some one pigeon showed a slight tendency to this 
strange habit, and that the long-continued selection of the best 
individuals in successive generations made tumblers what they now 
are ; and near Glasgow there are house-tumblers, as I hear from 
Mr. Brent, which cannot fly eighteen inches high without going 
head over heels. It may be doubted whether any one would have 
thought of training a dog to point, had not some one dog naturally 
shown a tendency in this line ; and this is known occasionally to 
happen, as I once saw, in a pure terrier : the act of pointing is pro- 
bably, as many have thought, only the exaggerated pause of an 
animal preparing to spring on its prey. When the first tendency 
to point was once displayed, methodical selection and the inherited 
efi'ects of compulsory training in each successive generation would 
soon complete the work ; and unconscious selection is still in 
progress, as each man tries to procure, without intending to improve 
the breed, dogs which stand and hunt best. On the other hand, 
habit alone in some cases has sufficed ; hardly any animal is more 


difficult to tame than the young of the wild rabbit ; scarcely any 
animal is tamer than the young of the tame rabbit; but I can 
hardly suppose that domestic rabbits have often been selected for 
tameness alone ; so that we must attribute at least the greater part 
of the inherited change from extreme wildness to extreme tame- 
ness, to habit and long-continued close confinement. 

Natural instincts are lost under domestication: a remarkable 
instance of this is seen in those breeds of fowls which very rarely 
or never become " broody," that is, never wish to sit on their eggs. 
Familiarity alone prevents our seeing how largely and how perma- 
nently the minds of our domestic animals have been modified. It 
is scarcely possible to doubt that the love of man has become 
instinctive in the dog. All wolves, foxes, jackals, and species of 
the cat genus, when kept tame, are most eager to attack poultry, 
sheep, and pigs; and this tendency has been found incurable in 
dogs which have been brought home as puppies from countries such 
as Tierra del Fuego and Australia, where the savages do not keep 
these domestic animals. How rarely, on the other hand, do our 
civilised dogs, even when quite young, require to be taught not to 
attack poultry, sheep, and pigs ! No doubt they occasionally do 
make an attack, and are then beaten ; and if not cured, they are 
destroyed; so that habit and some degree of selection have pro- 
bably concurred in civilising by inheritance our dogs. On the 
other hand, young chickens have lost, wholly by habit, that fear of 
the dog and cat which no doubt was originally instinctive in them ; 
for I am informed by Captain Button that the young chickens of 
the parent-stock, the Gallus bankiva, when reared in India under a 
hen, are at first excessively wild. So it is with young pheasants 
reared in England under a hen. It is not that chickens have lost 
all fear, but fear only of dogs and cats, for if the hen gives the 
danger-chuckle, they will run (more especially young turkeys) from 
Tinder her, and conceal themselves in the surrounding grass or 
thickets ; and this is evidently done for the instinctive purpose of 
allowing, as we see in wild ground-birds, their mother to fly away. 
But this instinct retained by our chickens has become useless under 
domestication, for the mother-hen has almost lost by disuse the 
power of flight. 

Hence, we may conclude, that under domestication instincts have 
been acquired, and natural instincts have been lost, partly by habit, 
and partly by man selecting and accumulating, during successive 
generations, peculiar mental habits and actions, which at first 
appeared from what we must in our ignorance call an accident. In 
some cases compulsory habit alone has sufficed to produce inhe- 

p 2 


rited mental changes; in other cases compulsory habit has done 
nothing, and all has been the result of selection, pursued both 
methodically and unconsciously: but in most cases habit and 
eelection have probably concurred. 

Special Instincts. 

We shall, perhaps, best understand how instincts in a state of 
nature have become modified by selection, by considering a few 
cases. 1 will select only three,— namely, the instinct which leads 
the cuckoo to lay her eggs in other birds' nests ; the slave-making 
instinct of certain ants ; and the cell-making power of the 
hive- bee. These two latter instincts have generally and justly 
been ranked by naturalists as the most wonderful of all known 

Instincts of the CucTcoo. — It is supposed by some naturalists that 
the more immediate cause of the instinct of the cuckoo is, that she 
lays her eggs, not daily, but at intervals of two or three days ; so 
that, if she were to make her own nest and sit on her own eggs, 
those first laid would have to be left for some time unincubated, or 
there would be eggs and young birds of different ages in the sama 
nest. If this were the case, the process of laying and hatching 
might be inconveniently long, more especially as she migrates at a 
very early period ; and the first hatched young would probably 
have to be fed by the male alone. But the American cuckoo is in 
this predicament ; for she makes her own nest, and has eggs and 
young successively hatched, all at the same time. It has been both 
asserted and denied that the American cuckoo occasionally lays hei 
eggs in other birds' nests ; but I have lately lieard from Dr. Merrell, 
of Iowa, that he once found in Illinois a young cuckoo together 
with a young jay in the nest of a Blue jay (Garrulus cristatus) ; 
and as both were nearly fully feathered, there could be no mistake 
in their identification. I could also give several instances of various 
birds which have been known occasionally to lay their eggs in other 
birds' nests. Kow let us suppose that the ancient progenitor of our 
European cuckoo had the habits of the American cuckoo, and that 
she occasionally laid an egg in another bird's nest. If the old bird 
profited by this occasional habit through being enabled to migrate 
earlier or through any other cause ; or if the young were made more 
vigorous by advantage being taken of the mistaken instinct of 
another species than when reared by their own mother, encum- 
bered as she could hardly fail to be by having eggs and young of 
different ages at the same time ; then the old birds or the fostered 
ycung would gain an advantage. And analogy would lead us tc 


believe, that the young thus reared would be apt to follow by inhe- 
ritance the occasional and aberrant habit of their mother, and in 
their turn would be apt to lay their eggs in other birds' nests, and 
thus be more successful in rearing their young. By -a continued 
process of this nature, I believe that the strange instinct of our 
cuckoo has been generated. It has, also, recently been ascertained 
on sufficient evidence, by Adolf Mliller, that the cuckoo occasionally 
lays her eggs on the bare ground, sits on them, and feeds her young. 
This rare event is probably a case of reversion to the long-lost, 
aboriginal instinct of nidification. 

It has been objected that I have not noticed ether related instincts 
and adaptations of structure in the cuckoo, which are spoken of as 
necessarily co-ordinated. But in all cases, speculation on an instinct 
known to us only in a single species, is useless, for we have hitherto 
had no facts to guide us. Until recently the instincts of the Euro- 
pean and of the non-parasitic American cuckoo alone were known ; 
now, owing to Mr. Eamsay's observations, we have learnt something 
about three Australian species, which lay their eggs in other birds' 
nests. The chief points to be referred to are three : first, that the 
common cuckoo, with rare exceptions, lays only one egg in a nest, 
so that the large and voracious young bird receives ample food, 
Secondly, that the eggs are remarkably small, not exceeding those 
of the skylark, — a bird about one-fourth as large as the cuckoo. 
That the small size of the egg is a real case of adaptation we may 
infer from the fact of the non-parasitic American cuckoo laying 
full-sized eggs. Thirdly, that the young cuckoo, soon after birth, 
has the instinct, the strength, and a properly shaped back for 
<3iecting. i^sj oster-brothers , which then perish from cold and hunger. 
This has been boldly called a beneficent arrangement, in order that 
the young cuckoo may get sufficient ]oo3,^and that its foster-brothers 
may pei-ish before J.liey had acquired much feeling! 

Turning now to the Australian species ; though these birds gene- 
rally lay only one egg in a nest, it is not rare to fi.nd two and even 
three eggs in the same nest. In the Bronze cuckoo the eggs vary 
greatly in size, from eight to ten lines in length. Now if it had 
been of an advantage to this species to have laid eggs even smaller 
than those now laid, so as to have deceived certain foster-parents, 
or, as is more probable, to have been hatched within a shorter period 
(for it is asserted that there is a relation between the size of eggs 
and the period of their incubation), then there is no difficulty in 
believing that a race or species might have been formed which 
would have laid smaller and smaller eggs ; for these would have 
been more safely hatched and reared. Mr. Kamsay remarks that 







^^ \>cj/^-i- 


[Chap. VI 11. 

two of the Australian cuckoos, when they lay their eggs in an open 
nest, manifest a decided preference for nests containing eggs similar 
in colour to their own. The European species apparently manifests 
some tendency towards a similar instinct, but not rarely departs 
from it, as is shown by her laying her dull and pale-coloured eggs 
in the nest of the Hedge- warbler with bright greenish- blue eggs. Had 
our cuckoo invariably displayed the above instinct, it would assu- 
tedly have been added to those which it is assumed must all have 
been acquired together. The eggs of the Australian Bronze cuckoo 
vary, according to Mr. Ramsay, to an extraordinary degree in colour; 
so that in this respect, as well as in size, natural selection might 
have secured and fixed any advantageous variation. 

In the case of the European cuckoo, the offspring of the foster- 
parents are commonly ejected from the nest within three days after 
the cuckoo is hatched ; and as the latter at this age is in a most 
helpless condition, Mr. Gould was formerly inclined to believe that 
the act of ejection was performed by the foster-parents themselves. 
But he has now received a trustworthy account of a young cuckoo 
which was actually seen, whilst still blind and not able even to 
hold up its own head, in thea£tof_ejectingjjts_fcfiter^^ One 

of these was replaced in tKe^nest by the observer, and, was agam 
thrown out. With respect to the means by which this strange and 
odious instinct was acquired, if it were of great importance for the 
young cuckoo, as is probably the case, to receive as much food a9 
possible soon after birth, I can see no special difficulty in its having 
gradually acquired, during successive generations, the ^bjiftd. desire,,^ 
the strength, and structure necessary for the work of ejection ; for 
those young cuckoos which had such habits and structure best deve- 
loped would be the most securely reared. The first step towards 
the acquisition of the proper instinct might have been mere unin- 
tentional restlessness on the part of the young bird, when somewhat 
advanced in age and strength ; the habit having been afterwards 
improved, and transmitted to an earlier age. I can see no more 
difficulty in this, than in the unhatched young cf other birds ac- 
•quiring the instinct to break through their own shells ; — or than in 
young snakes acquiring in their upper jaws, as Owen has remarked, 
a transitory sharp tooth for cutting through the tough egg-shelU 
For if each part is liable to individual variations at all ages, and the 
variations tend to be inherited at a corresponding or earlier age, — • 
propositions which cannot be disputed, — then the instincts and 
structure of the young could be slowly modified as surely as thor.e ol 
the adult ; and both cases must stand or fall together with the whole 
theory of natural selection. 


Some species of Molothrus, a widely distinct genus of American 
birds, allied to our starlings, have parasitic habits like those of the 
cuckoo ; and the species present an interesting gradation in the per- 
fection of their instincts. The sexes of Molothrus badius are stated 
by an excellent observer, Mr. Hudson, sometimes to live promis- 
cuously together in flocks, and sometimes to pair. They either 
build a nest of their own, or seize on one belonging to some other 
bird, occasionally throwing out the nestlings of the stranger. They 
either lay their eggs in the nest thus appropriated, or oddly enough 
build one for themselves on the top of it. They usually sit on 
their own eggs and rear their own young ; but Mr. Hudson says 
it is probable that they are occasionally parasitic, for he has seen 
the young of this species following old birds of a distinct kind 
and clamouring to be fed by them. The parasitic habits of another 
species of Molothrus, the M. bonariensis, are much more highly 
developed than those of the last, but are still far from perfect. 
This bird, as far as it is known, invariably lays its eggs in the 
nests of strangers ; but it is remarkable that several together 
sometimes commence to build an irregular untidy nest of their 
own, placed in singularly ill-adapted situations, as on the leaves 
of a large thistle. They never, however, as far as Mr. Hudson 
has ascertained, complete a nest for themselves. They often lay 
so many eggs — from fifteen to twenty — in the same foster-nest, 
that few or none can possibly be hatched. They have, moreover, 
the extraordinary habit of pecking holes in the eggs, whether oi 
their own species or of their foster-parents, which they find in the 
appropriated nests. They drop also many eggs on the bare ground, 
which are thus wasted. A third species, the M. pecoris of North 
America, has acquired instincts as perfect as those of the cuckoo, 
for it never lays more_thajQjone egg_in.a foster^ 
young bird is securely reared . Mr. Hudson is a strong disbeliever 
in evolution, but he appears to have been so much struck by the 
imperfect instincts of the Molothrus bonariensis that he quotes my 
words, and asks, " Must we consider these habits, not as especially 
endowed or created instincts, but as small consequences of one 
general bw, namely, transition?" 

Various birds, as has already been remarked, occasionally lay 
their eggs in the nests of other birds. This habit is not very 
uncommon with the Gallinaceje, and throws some light on the 
singular instinct of the ostrich. In this family several hen-birds 
unite and lay first a few eggs in one nest and then in another ; and 
these are hatched by the males. This instinct may probably be 
accounted for by the fact of the hens laying a large number of eggs, 


but, as with the cuckoo, at intervals of two or three days. The 
instinct, however, of the American ostrich, as in the case of the Mol- 
othrus bonariensis, has not as yet been perfected ; for a surprising 
number of eggs lie strewed over the plains, so that in one day's 
hunting I picked up no less than_twentjr JjDstjmd wasted_eggs. 

Many bees are parasitic, and regularly lay their eggs in the nests 
of other kinds of bees. This case is more remarkable than that of 
the cuckoo ; for these bees have not only had their instincts but 
their structure modified in accordance with their parasitic habits ; 
for they do not possess the pollen-collecting apparatus which would 
have been indispensable if they had stored up food for their own 
young. Some species of Sphegida^ (wasp-like insects) are likewise 
parasitic ; and M. Fabre has lately shown good reason for believing 
that, although the Tachytes nigra generally makes its own burrow 
and stores it with paralysed prey for its own larva?, yet that, wdien 
this insect finds a T)iirrow~ already made and stored by another 
sphex, it takes advantage of the prize, and becomes for the occasion 
parasitic. In this case, as with that of the Molothrus or cuckoo, I 
can see no difficulty in natural selection making an occasional habit 
permanent, if of advantage to the species, and if the insect whose 
nest and stored food are feloniously appropriated, be not thus exter- 

Slave-making instinct. — This remarkable instinct was first dis- 
covered in the Formica (Polyerges) rufescens by Pierre Huber, a 
better observer even than his celebrated father. This ant is abso- 
lutely dependent on its slaves ; without their aid, the species would 
certainly become extinct in a single year. The males and fertile 
females do no work of any kind, and the workers or sterile females, 
though most energetic and courageous in capturing slaves, do no 
other work. They are incapable of making their own nests, or of 
feeding their own larvse. When the old nest is found inconvenient, 
and they have to migrate, it is the slaves which determine the 
migration, and actually carry their masters in their jaws. So utterly 
helpless are the masters, that when Huber shut up thirty of them 
without a slave, but with plenty of the food which they like best, 
and with their own larvae and pupse to stimulate them to work, they 
did nothing; they could not even feed themselves, and many perished 
of hunger. Huber then introduced a single slave (F. lusca), and she 
instantly set to work, fed and saved the survivors ; made some cells 
and tended the larvse, and put all to rights. What can be more extra- 
ordinary than these well-ascertained facts ? If we had not known of 
any other slave-making ant, it would have been hopeless to FpecnJate 
how so wonderful an instinct could have been perfected. 


Another species, Formica sanguinea, was likewise first discovered 
by P. Huber to be a slave-making ant. This species is found in 
the southern parts of England, and its habits have been attended 
to by Mr. F. Smith, of the British Museum, to whom I am much 
indebted for information on this and other subjects. Althou^^h 
fully trusting to the statements of Huber and Mr. Smith, I tried to 
approach tbe subject in a sceptical frame of mind, as any one may 
well be excused for doubting the existence of so extraordinary an 
instinct as that of making slaves. Hence, I will give the observa- 
tions which I made, in some little detail. I opened fourteen nests 
of F. sanguinea, and found a few slaves in all. Males and fertile 
females of the slave species (F. fusca) are found only in their own 
proper communities, and have never been observed in the nests oi 
F. sanguinea. The slaves are_black and not above half the size 
of theirredmaste/s, so that the contrast in their a^)earancelir 
great when tlTe'nest is slightly disturbed, the slaves occasionally 
come out, and like their masters are much agitated and defend the 
nest : when the nest is much disturbed, aud the larvae and pup^e 
are exposed, the slaves work energetically together with their 
masters in carrying them away to a place of safety. Hence, it is 
clear, that the slayes.feel quite at home. During the months ot 
June and July, on three successive years, I watched for many hours 
several nests in Surrey and Sussex, and never saw a slave either 
leave or enter a nest. As, during these months, the slaves are very 
few in number, I thought that they might behave differently when 
more numerous ; but Mr. Smith informs me that he has watched 
the nests at various hours during May, June, and August, both in 
Surrey and Hampshire, and has never seen the slaves, though 
present in large numbers in August, either leave or enter the nest. 
Hence he considers them as strictly household slaves. The masters, 
on the other hand, may be constantly seen bringing in materials for 
the nest, and food of all kinds. During the year 1860, however, 
in the month of July, I came across a community with an unusually 
large stock of slaves, and I observed a few slaves mingled with 
their masters leaving the nest, and marching along the same road 
to a tall Scotch-fir-tree, twenty-five yards distant, which they 
ascended together, probably in search of aphides or cocci. According 
to Huber, who had ample opportunities for observation, the slaves 
in Switzerland habitually work with their masters in making the 
nest, and they alone open and close the doors in the morning and 
evening; and, as Huber expressly states, their" principal office is 
to^^s^rch for aphides. This difference in the usual habits of the 
lijasters and slaves in the two countries, probably depends merely 


on the slaves being captured in greater numbers in Switzerland 
than in England. 

One day I fortunately witnessed a migration of F. sanguinea from 
one nest to another, and it was a most interesting spectacle to 
behold the masters carefully carrying their slaves in their jaws 
instead of being carried by them, as in the case of F. rufescens 
Another day my attention was struck by about a score of the slave- 
makers haunting the same spot, and evidently not in search of 
food ; they approached and were vigorously repulsed by an inde- 
pendent community of the slave-species (F. fusca) ; sometimes as 
many as three of these ants clinging to the legs of the slave-making 
F. sanguinea. The latter ruthlessly killed their small opponents, 
and carried their dead bodies as food to their nest, twenty-nine yards 
distant; but they were prevented from getting any^pupseTo" rear 
as slaves. I then dug up a small parcel of the pupas of F. fusca 
from another nest, and put them down on a bare spot near the 
place of combat ; they were eagerly seized and carried off by the 
tyrants, who perhaps fancied that, after all, they had been victorious 
in their late combat. 

At the same time I laid on the same place a small parcel of the 
pupas of another species, F. flava, with a few of these little yellow 
ants still clinging to the fragments of their nest. This species is 
sometimes, though rarely, made into slaves, as has been described 
by Mr. Smith. Although so small a species, it is very courageous, 
and I have seen it ferociously attack other ants. In one instance 
I found to my surprise an independent community of F. flava 
under a stone beneath a nest of the slave-making F. sanguinea ; 
and when I had accidentally disturbed both nests, the little ants 
attacked their big neighbours with surprising courage. Now I was 
curious to ascertain whether F. sanguinea could distinguish the 
pup£e of F. fusca, which they habitually make into slaves, from 
those of the little and furious F. flava, which they rarely capture, 
and it was evident that they did _at once distinguish them ; for 
we have seen that they eagerly and instantly seized the pupaj of 
F. fusca, whereas they were much terrified when they came across 
the pupa3, or even the earth from the nest, of F. flava, and quickly 
ran away ; but in about a quarter of an hour, shortly after all the 
little yellow ants had crawled away, they took heart and carried 
off the pup£e. 

One evening I visited another community of F. sanguinea, and 
found a number of these ants returning home and entering their 
nests, carrying the dead bodies of F. fusca (showing that it w&s not 
a migration) and numerous pupse. I traced a long file ol ants 


burthened withjDOotj, for about tjortj yards back, to a very thick 
clump of heath, whence I saw the last individual of" F. sanguinea 
emerge, carrying a pupa ; but T was not able to find the desolated 
nest in the thick heath. The nest, however, must have been close 
at hand, for two or three individuals of F. fusca were rushing about 
in the greatest agitation, and one was perched motionless with its 
own pupa in its mouth on the top of a spray of heath, an image 
of despair over its ravaged home. 

Such are the facts, though they did not need confirmation by me, 
in regard to the wonderful instinct of making slaves. Let it be 
observed what a contrast the instinctive habits of F. sanguinea 
present with those of the continental F. rufescens. The latter does 
not build its own nest, does not determine its own migrations, does 
not collect food for itself or its young, and cannot even feed itself : 
it is absolutely dependent_on_Jts numerous slaves. Formica san- 
guinea, on the other hand, possesses much fewer slaves, and in the 
early part of the summer extremely few : the masters determine 
when and where a new nest shall be formed, and when they 
migrate, the masters carry the slaves. Both in Switzerland and 
England the slaves seem to have the exclusive care of the larvse, 
and the masters aJone go on slave-making expeditions. In Switzer- 
land the slaves and masters work together, making and bringing 
materials for the nest; both, but chiefly the slaves, tend, and 
milk, as it may be called, their aphides ; and thus both collect 
food for the community. In England the masters alone usually 
leave the nest to collect building materials and food for themselves, 
their slaves and larvae. So that the masters in this country receive 
much less service from their slaves than they do in Switzerland. 

By what steps the instinct of F. sanguinea originated I will not 
pretend to conjecture. But as ants, which are not slave-makers 
will, as I have seen, carry off the pupse of other species, if scattered 
near their nests, it is possible that such pup^e originally stored as 
food might become developed ; and the foreign ants thus uninten- 
tionally reared would then follow their proper instincts, and do what 
work they could. If their presence proved useful to the species 
which had seized them — if it were more advantageous to this 
species to capture workers than to procreate them — the habit of 
collecting pupae, originally for food, might by natural selection be 
strengthened and rendered permanent for the very different purpose 
of raising slaves. When tb3 instinct was once acquired, if carried 
out to a much less extent 3ven than in our British F. sanguinea, 
which, as we have seen, is less aided by its slaves than the same 
Epecies in Switzerland, natural selection might increase and modify 


the instinct — always supposing each modification to be of use to the 
species —until an ant was formed as abjectly dependent on its 
slaves as is the Formica rufescens. 

Cell-making instinct of the Hive-Bee. — I will not here enter on 
minute details on this subject, but will merely give an outline ol 
the conclusions at which I have arrived. He must be a dull man 
who can examine the exquisite structure of a comb, so beautifully 
adapted to its end, without enthusiastic admiration. We hear from 
mathematicians that'l)ees have practically^solyed a recondite pro- 
blem, and have made their cells of the proper shape To hold tlie 
greatest possible amount of honey, with the least possible coa.:^ 
sumption of precious wax in their construction. It has been re- 
marked that a skilful workman with fitting tools and measures, 
would find it very difficult to make cells of wax of the true form, 
though this is efi"ected by a crowd of bees working in a dark hive. 
Granting whatever instincts you please, it seems at first quite 
inconceivable how they can make all the necessary angles and 
planes, or even perceive when they are correctly made. But the 
difiiculty is not nearly so great as it at first appears : all this 
beautiful work can be shown, I think, to follow from a few simple 

I was led to investigate this subject by Mr. Waterhouse, who has 
shown that the form of the cell stands in close relation to the 
presence of adjoining cells ; and the following view may, perhaps, 
be considered only as a modification of his theory. Let us look 
to the great principle of gradation, and see whether Nature does 
not reveal to us her methoH^oTwork. At one end of a short series 
we have humble-bees, which use their ol d cocoons to hold_honey, 
sometimes adding to them short tubes of wax, and li^wise making 
separate and very irregular rounded cells of wax. At the other end 
of the series we have the cells of the hive-bee, placed in a double 
layer: each cell, as is well known, is an hexagonal prism, with 
the basal edges of its six sides bevelled so as to join an inverted 
pyramid, of three rhombs. These rhombs have certain angles, and 
the three which form the pyramidal base of a single cell on one 
side of the comb enter into the composition of the bases of three 
adjoining cells on the opposite side. In the series between the 
extreme perfection of the cells of the hive-bee and the simplicity 
of those of the hnmble-bee we have the cells of the Mexican 
Melipona domestica^ carefully described and figured by Pierre Huber. 
The Melipona itself is intermediate in structure between the hive 
and hrimble bee, but more nearly related to the latter ; it fcrms a 
nearly regular waxen comb of cylindrical cells, in which the young 


are hatclied, and, in addition, some large cells of wax foi holding 
honey. These latter cells are nearly spherical and of neaily equal 
sizes, and are aggregated into an irregular mass. But the im- 
portant point to notice is, that these cells are always made at that 
degree of nearness to each other that they would have intersected 
or broken into each other if the spheres had _been completed; but 
this is never permitted, the bees building perfectly flat 
between the spheres whichjhiia_tend to intersect ._Hence, each cell 
consists of an outer spherical portion, and of two, three, or more flat 
surfaces, according as the cell adjoins two, three, or more other "-^^-^^ 
cells. When one cell rests on three other cells, which, from the 1^ 
spheres being nearly of the same size, is very frequently and neces- ,-— *^v— n^ 
sarily the case, the three flat surfaces are united into aj ryramid ; T | 

and this pyramid, as Huber has remarked, is manifestly a gross irni- /^ ' 
tation of the three-sided pyramidal base of the cell of the hive-bee. 
-As in the cells of the hive-bee, so here, the three plane surfaces in 
any one cell necessarily enter into the construction of three adjoin- 
ing cells. It is obvious that the Melipona saves wax, and what 
is more important, labour, by this manner of building ; for the flat 
walls between the adjoining cells are not double, but are of the 
same thickness as the outer spherical portions, and yet each flat 
portion forms a part of ^two cells. 

Reflecting on this case, irbccurred to me that if the Melipona 
had made its spheres at some given distance from each other, and 
had made them of equal sizes and had arranged them symmetrically 
in a double layer, the resulting structure would have been as per- 
fect as the comb of the hive-bee. Accordingly I wrote to Professor 
Miller, of Cambridge, and this geometer has kindly read over the 
following statement, drawn up from his information, and tells me 
that it is strictly correct : — 

If a number of equal spheres be described with their centres 
placed in two parallel layers ; with the centre of each sphere at the 
distance of radius x >v/ 2, or radius x 1*41421 (or at some lesser 
distance), from the centres of the six surrounding spheres in the 
same layer; and at the same distance from the centres of the ad- 
joining spheres in the other and parallel layer ; then, if planes of 
intersection between the several spheres in both layers be formed, 
there will result a double layer^o^hexagonal^lsms united together 
by pyramidal bases formed of three~rhomBs ; and the rhombs and 
ihe sides of the hexagonal prisms will have every angle identically 
the same with the best measurements which have been made of the 
cells of the hive-bee. But I hear from Prof. Wyman, who has 
made numerous careful mea? urements, that the accura«Tof the 


workmanship of the bee has been greatly exaggerated ; so much so, 
that whatever t^^p- tj 7 Tj j]{^l^ ^^^^ of the cell ma y be, i t is rarely, if 
ever, realised. 

Hence we may safely conclude that, if we could slightly modify 
the instincts already possessed by the Melipona, and in themselves 
not very wonderful, this bee would make a structure as wonderfully 
perfect as that of the hive-bee. We must sapposc the Melipona to 
have the power of forming her cells truly spherical, and of equal 
sizes ; and this would not be very surprising, seeing that she already 
does so to a certain extent, and seeing what perfectly cylindrical 
burrows many insects make in wodd, apparently by turning round 
on a fixed point. We must suppose the Melipona to arrange her 
cells in level layers, as she already does her cylindrical cells ; and 
we must further suppose, and this is the greatest difficulty, that 
she can somehow judge accurately at what distance to stand from 
her fellow-labourers when several are making their spheres ; but 
she is already so far enabled to judge of distance, that she always 
describes her spheres so as to intersect to a certain extent ; and then 
she unites the points of intersection by perfectly fiat surfaces. By 
such modifications of instincts which in themselves are not very 
wonderful, — hardly more wonderful than those which guide a 
bird to make its nest, — 1 believe that the hive-bee has acquired, 
through natural selection, her inimitable architectural powers. 

But this theory can be tested by experiment. Following the 
example of Mr. Tegetmeier, I separated two combs, and put between 
them a long, thick, rectangular strip of wax: the bees instantly 
began to excavate minute circular pits in it ; and as they deepened 
these little pits, they made them wider and wider until they were 
converted into shallow basins, appearing to the eye perfectly true or 
parts of a sphere, and of about the diameter of a cell. It was most 
interesting to observe that, wherever several bees had begun to 
excavate these basins near together, they had begun their work at 
such a distance from each other, that by the time the basins had 
acquired the above-stated width (i. e. about the width of an ordinary 
cell), and were in depth about one sixth of the diameter of the 
sphere of which they formed a part, the rims of the basins intersected 
or broke into each other. As soon as this occurred, the bees ceased 
to excavate, and began to build up flat walls of wax on the lines of 
intersection between the basins, so that each hexagonal prism was 
built upon the scalloped edge of a smooth basin, instead of on the 
straight edges of a three-sided pyramid as in the case of oidlnary 

I then put into the hive, instead of a thick, rectangular piece of 


wax, a thin and narrow, knife-edged ridge, coloured with vermilion. 
The bees instantly began on both sides to excavate little basins 
near to each other, in the same way as before ; but the ridge of wax 
was so thin, that the bottoms of the basins, if they had been exca- 
vated to the same depth as in the former experiment, would have 
broken into each other from the opposite sides. The bees, however, 
did not suffer this to happen, and they stopped their excavations in 
due time; so that the basins, as soon as they had been a littln 
deepened, came to have flat bases ; and these fiat bases, formed by 
thin little plates of the vermilion wax left ungnawed, were situated^ 
as far as the eye could judge, exactly^ along the planes of imaginary 
intersection between the basins on the opposite sides of the ridge 
of wax. In "some parts, only small portions, in other parts, large 
portions of a rhombic plate were thus left between the opposed 
basins, but the work, from the unnatural state of things, had not 
been neatly performed. The bees must have worked at very 
nearly the same rate in circularly gnawing away and deepening 
the basins on both sides of the ridge of vermilion wax, in order to 
have thus succeeded in leaving flat plates between the basins, by 
stopping work at the planes of intersection. 

Considering how flexible thin wax is, I do not see that there is 
any difficulty in the bees, whilst at work on the two sides of a strip 
)f wax, perceiving when they have gnawed the wax away to the 
proper thinness, and then stopping their work. In ordinary combs 
t has appeared to me that the bees do not always succeed in work- 
ing at exactly the same rate from the opposite sides ; for I have 
noticed half-completed rhombs at the base of a just-commenced cell, 
which were slightly concave on one side, where I suppose that the 
bees had excavated too quickly, and convex on the opposed side 
where the bees had worked less quickly. In one well marked 
instance, I put the comb back into the hive, and allowed the bees 
to go on working for a short time, and again examined the cell, and 
I found that the rhombic plate had been completed, and had become 
perfectly flat : it was absolutely impossible, from the extreme thin- 
ness of the Uttle plate, that they could have effected this by gnawing 
away the convex side ; and I suspect that the bees in such cases 
stand on opposite sides and push and bend the ductile and warm 
wax (which as I have tried is easily done) into its proper inter- 
mediate plane, and thus flatten it. 

From the experiment of the ridge of vermilion wax we can see 
that, if the bees were to build for themselves a thin wall of wax, 
they could make their cells of the proper shape, by standing at the 
proper distance from each other, by excavating at the same rate. 


and by endeavouring to make equal spherical hollows, but never 
allowing the spheres to break into each other. Kow bees, a-s may * 
be clearly seen by examining the edge of a growing comb,'do make 
a rough, circumferential wall or rim all round the comb ; and they 
gnaw this away from the opposite sides, always working circularly 
as they deepen each cell. They do not make the whole three-sided 
pyramidal base of any one cell at the same time, but only that 
one rhombic plate which stands on the extreme growing margin, or 
the two plates, as the case may be ; and they never complete the 
upper edges of the rhombic plates, until the hexagonal walls are 
commenced. Some of these statements differ from those made by 
the justly celebrated elder Huber, but I am convinced of their 
accuracy ; and if I had space, I could show that they are conformable 
with my theory. 

Huber's statement, that the very first cell is excavated out of a 
little par.allel-sided wall of wax, is not, as far as I have seen, strictly 
correct ; the first commencement havmg always been a little hood 
of wax ; but I will not here enter on details. We see how important 
a part excavation plays in the construction of the cells; but it 
would be a great error to suppose that the bees cannot build up a 
rough wall of wax in the proper position — that is, along the plane 
of intersection between two adjoining spheres. I have several spe- 
cimens showing clearly that they can do this. Even in the rude 
circumferential rim or wall of wax round a growing comb, flexures 
may sometimes be observed, corresponding in position to the planes 
of the rhombic basal plates of future cells. But the rough wall of 
wax has in every case to be finished off, by being largely gnawed 
away on both sides. The manner in which the bees build is 
curious ; they always make the first rough wall from ten to twenty 
times thicker than the excessively thin finished wall of the cell, 
which will ultimately be left. "We shall understand how they 
work, by supposing masons first to pile up a broad ridge of cement, 
and then to begin cutting it away equally on both sides near the 
ground, till a smooth, very thin wall is left in the middle ; the 
masons always piling up the cut-away cement, and adding fresh 
cement on the summit of the ridge. We shall thus have a thin 
wall steadily growing upward but always crowned by a gigantic 
coping. From all the cells, both those just commenced and those 
completed, being thus crowned by a strong coping of wax, the bees 
can cluster and crawl over the comb without injuring the delicate 
hexagonal walls. These walls, as Professor Miller has kindly ascer- 
tained for me, vary greatly in thickness ; being, on an average of 
twelve measurements made near the border of the comb, j\^ of ar 


Iflch in thickness; whereas the basal rhomboidal plates are thicker, 
nearly in ths proportion of three to two, having a mean thickness 
from twenty-one measurements, of 2^ of an inch. By the above 
singular manner of building, strength is continually given to the 
comb, with the utmost ultimate economy of wax. 

It seems at first to add to the difficulty of understanding how 
the cells are made, that a multitude of bees all work together ; one 
bee after working a short time at one cell going to another, so that, 
as Huber has stated, a score of individuals work even at the com- 
mencement of the first cell. I was able practically to show this 
fact, by covering the edges of the hexagonal walls of a single cell, 
or the extreme margin of the circumferential rim of a growing 
comb, with an extremely thin layer of melted vermilion wax ; and 
I invariably found that the colour was most delicately diffused by 
the bees — as delicately as a painter could have done it with his brush 
— by atoms of the coloured wax having been taken from the spot 
on which it had been placed, and worked into the growing edges of 
the cells all round. The vrork of construction seems to be a sort 
of balance struck between many bees, all instinctively standing at 
the same relative distance from each other, all trying tO sweep equal 
spheres, and then building up, or leaving un gnawed, the planes of 
intersection between these spheres. It was really curious to note 
in cases of difficulty, as when two pieces of comb met at an angle, 
how often the bees would pull down and rebuild in different ways 
the same cell, sometimes recurring to a shape which they had at 
first rejected. 

When bees have a place on which they can stand in their proper 
positions for working, — for instance, on a slip of wood, placed 
directly under the middle of a comb growing downwards, so that 
the comb has to be built over one face of the sUp — in this case the 
bees can lay the foundations of one wall of a new hexagon, in its 
strictly proper place, projecting beyond the other completed cells. 
It suffices that the bees should be enabled to stand at their proper 
relative distances from each other and from the walls of the last 
completed cells, and then, by striking imaginary spheres, they can 
build up a wall intermediate between two adjoining spheres ; but, 
as far as I have seen, they never gnaw away and finish off the 
angles of a cell till a large part both of that cell and of the adjoin- 
ing cells has been built. This capacity in bees of laying down 
under certain circumstances a rough wall in its proper place between 
two just-commenced cells, is important, as it bears on a fact, which 
seems at first subversive of the foregoing theory ; namely, that the 
cells on the extreme, margin of wasp-combs are sometimes strictly 



hexagonal ; but I have not space here to enter on this subject. Nor 
does there seem to me any great difiSculty in a single insect (as in 
the case of a queen-wasp) making hexagonal cells, if she were to 
work alternately on the inside and outside of two or three cells com- 
menced at the same time, always standing at the proper relative 
distance from the parts of the cells just begun, sweeping spheres or 
cylinders, and building up intermediate planes. 

As natural selection acts only by the accumulation of slight 
modifications of structure or instinct, each profitable to the indi- 
vidual under its conditions of life, it may reasonably be asked, how 
a long and graduated succession of modified architectural instincts, 
all tending towards the present perfect plan of construction, could 
have profited the progenitors of the hive-bee ? I think the answer 
is not difficult : cells constructed like those of the bee or the wasp 
gain in strength, and save much in labour and space, and in the 
materials of which they are constructed. With respect to the for- 
mation of wax, it is known that bees are often hard pressed to get 
sufficient nectar, and I am informed by Mr. Tegetmeier that it has 
been experimentally proved that from twelve to fifteen pounds of 
dry sugar are consumed by a hive of bees for the secretion of a 
pound of wax ; so that a prodigious quantity of fluid nectar must 
be collected and consumed by the bees in a. hive for the secretion 
of the wax necessary for the construction of their combs. More- 
over, many bees have to remain idle for many days during the 
process of secretion. A large store of honey is indispensable to 
support a large stock of bees during the winter ; and the security 
of the hive is known mainly to depend on a large number of bees 
being supported. Hence the saving of wax by largely saying_liDney 
and the time consumed iii collecting the honey must be an import- 
ant element of success to any family of bees. Of course the success 
of the species may be dependent on the number of its enemies, or 
parasites, or on quite distinct causes, and so be altogether inde- 
pendent of the quantity of honey which the bees can collect. But 
let us suppose that this latter circumstance determined, as it pro- 
bably often has determined, whether a bee allied to our humble- 
bees could exist in large numbers in any country; and let us 
further suppose that the community lived through the winter, and 
consequently required a store of honey : there can in this case bo 
no doubt that it would be an advantage to our imaginary humble- 
bee, if a slight modification in her instincts led her to make her 
waxen cells near together, so as to intersect a little ; for a wall in 
common even to two adjoining cells would save some little labour 
and wajt. Hence it would continually be more and more advaa- 


tageous to our humble-bees, if they were to make their cells more 
and more regular, nearer together, and aggregated into a mass, like 
the cells of the Melipona ; for in this case a large part of the 
bounding surface of each cell would serve to bound the adjoining 
cells, and much labour and wax would be saved. Again, from the 
same cause, it would be advantageous to the Melipona, if she were 
to make her cells closer together, and more regular in every way 
than at present ; for then, as we have seen, the spherical surfaces 
would wholly disappear and be replaced by plane surfaces ; and the 
Melipona would make a comb as perfect as that of the hive-bee. 
Beyond this stage of perfection in architecture, natural selection 
could not lead ; for the comb of the hive-bee, as far as we can see, 
is absolutely perfect in economising labour and wax. 

Thus, as I believe, the most wonderful of all known instincts, 
that of the hive-bee, can be explained by natural selection having 
taken advantage of numerous, successive, slight modifications of 
simpler instinc ts ; natural selection having, by slow degrees, more 
and more perfectly led the bees to sweep equal spheres at a given 
distance from each other in a double layer, and to build up and 
excavate the wax along the planes of intersection; the bees, of 
course, no more knowing that they swept their spheres at one par- 
ticular distance from each other, than they know what are the 
several angles of the hexagonal prisms and of the basal rhombic 
plates ; the motive power of the process of natural selection having 
been the construction of cells of due strength and of the proper 
size and shape for the larv£e, this being effected with the greatest 
possible economy of labour and wax ; that individual swarm which 
thus made the best cells with least labour, and least waste of honey 
in the secretion of wax, having succeeded best, and having trans- 
mitted their newly-acquired economical instincts to new swarms, 
which in their tm'n will have had the best chance of succeeding in 
the struggle for existence. 

Ohjections to the Theory of Natural Selection as ajpplied to Instincts : 
Neuter and Sterile Insects. 

It has been objected to the foregoing view of the origin of instincts 
that " the variations of structure and of instinct must have been 
simultaneous and accurately adjusted to each other, as a modifica- 
tion in the one without an immediate corresponding change in the 
other would have been fatal." The force of this objection rests 
entirely on the assumption that the changes in the instincts and 
structure are abrupt. To take as an illustration the case of the 
larger titmouse (Parus major) alluded to in a previous chapter; 

Q 2 


this bird often holds the seeds of the yew between its feet on a 
branch, and hammers with its beak till it gets at the kernel. Now 
what special difficulty would there be in natural selection preserving 
all the slight individual variations in the shape of the beak, which 
were better and better adapted to break open the seeds, until a 
beak was formed, as well constructed for this purpose as that of 
the nuthatch, at the same time that habit, or compulsion, or spon- 
taneous variations of taste, led the bird to become more and more 
of a seed-eater ? In this case the beak is supposed to be slowly 
modified by natural selection, subsequently to, but in accordance 
with, slowly changing habits of 'tasteYl)ut let the feet of the tit- 
mouse vary and grow larger from correlation with the beak, or 
from any other unknown cause, and it is not improbable that such 
larger feet would lead the bird to climb more and more until it 
acquired the remarkable cUmbing instinct and power of the nut- 
hatch. In this case a gradual change of structure is supposed to 
lead to changed instinctive habits. To take one more case : few 
instincts are more remarkable than that which leads the swift of 
the Eastern Islands to make its nest wholly ol jnspissatfid^saliva. 
Some birds build their nests of mud, believed to be moistened with 
saliva ; and one of the swifts of North America makes its nest (as 
I have seen) of sticks agglutinated with saliva, and even with flakes 
of this substance. Is it then very improbable that the natural 
selection of individual swifts, which secreted more and more saliva, 
should at last produce a species with instincts leading it to neglect 
other materials, and to make its nest exclusively of inspissated 
saliva? And so in other cases. It must, however, be admitted 
that in many instances we cannot conjecture whether it was instinct 
or structure which first varied. 

No doubt many instincts of very difficult explanation could be 
opposed to the theory of natural selection — cases, in which we can- 
not see how an instinct could have originated ; cases, in which no 
intermediate gradations are known to exist ; cases of instinct of 
such trifiing importance, that they could hardly have been acted on 
by natural selection ; cases of instincts almost identically the same 
in animals so remote in the scale of nature, that we cannot account 
for their similarity by inheritance from a common progenitor, and 
consequently must believe that they were independently acquired 
through natural selection. I will not here enter on these several 
cases, but will confine myself to one special difficulty, which dt first 
appeared to me insuperable, and actually fatal to the whole theory. 
I allude to the neuters or sterile females in insect-communities; 
for these neuters often differ widelv in instinct and in structure 


from both the males and fertile females, and yet, from being sterile, 
they cannot propagate their kind. 

The subject well deserves to be discussed at great length, but I 
will here take only a single case, that of working or sterile ants. 
How the workers have been rendered sterile is a difficulty ; but not 
much greater than that of any other striking modification of struc- 
ture ; for it can be shown that some insects and other articulate 
animals in a state of nature occasionally become sterile; and if 
such insects had been social, and it had been profitable to the com- 
munity that a number should have been annually bom capable of 
work, but incapable of procreation, I can see no especial difficulty 
in this having been eff'ected through natural selection. But I must 
pass over this preliminary difficulty. The great difficulty lies in 
the working ants differing widely from both the males and the 
fertile females in structure, as in the shape of the thorax, and in 
being destitute of wings and sometimes of eyes, and in instinct. 
As far as instinct alone is concerned, the wonderful difference in 
this respect between the workers and the perfect females, would 
have been better exemplified by the hive-bee. If a working ant or 
other neuter insect had been an ordinary animal, I should have 
unhesitatingly assumed that all its characters had been slowly 
acquired through natural selection ; namely, by individuals having 
been born with slight profitable modifications, which were inherited 
by the offspring; and that these again varied and again were 
selected, and so onwards. But with the working ant we have an 
insect diff'ering greatly from its parents, yet absolutely sterile ; so 
that it could never have transmitted successively acquired modifica- 
tions of structure or instinct to its progeny. It may well be asked 
how is it possible to reconcile this case with the theory of natural 
selection ? 

First, let it be remembered that we have innumerable instances, 
both in our domestic productions and in those in a state of nature, 
of aU sorts of differences of inherited structure which are correlated 
with certain ages, and with either sex. We have differences corre- 
lated not only with one sex, but with that short period when the 
reproductive system is active, as in the nuptial plumage of many 
birds, and in the hooked jaws of the male salmon. We have even 
slight differences in the horns of different breeds of cattle in rela- 
tion to an artificially imperfect state of the male sex ; for oxen oi 
o.ertain breeds have longer horns than the oxen of other breeds, 
relatively to tne length of the horns in both the bulls and cows of 
these same breeds. Hence I can see no great difficulty in any 
character becoming; correlated with the sterile condition of certaia 


members of insect-communities : the difficulty lies in understanding 
how such correlated modifications of structure could have been 
slowly accumulated by natural selection. 

This difficulty, though appearing insuperable, is lessened, or, as 
I believe, disappears, when it is remembered that selection may be 
applied to the family, as well as to the individual, and may thus 
gain the desired end. Breeders of cattle wish the flesh and fat to 
be well marbled together : an animal thus characterised has been 
slaughtered, but the breeder has gone with confidence to the same 
stock and has succeeded. Such faith may be placed in the power 
of selection, that a breed of cattle, always yielding oxen with extra- 
ordinarily long horns, could, it is probable, be formed by carefully 
watching which individual bulls and cows, when matched, produced 
oxen with the longest horns ; and yet no one ox would ever have 
propagated its kind. Here is a better and real illustration : accord- 
ing to M. Verlot, some varieties of the double annual Stock from 
having been long and carefully selected to the right degree, always 
produce a large proportion of seedlings bearing double and quite 
sterile flowers; but they likewise yield some single and fertile 
plants. These latter, by which alone the variety can be propagated, 
may be compared with the fertile male and female ants, and tlio 
double sterile plants with the neuters of the same community. 
As with the varieties of the stock, so with social insects, selection 
has been applied to the family, and not to the individual, for the 
sake of gaining a serviceable end. Hence we may conclude that 
slight modifications of structure or of instinct, correlated with the 
sterile condition of certain members of the community, have proved 
advantageous : consequently the fertile males and females have 
flourished, and transmitted to their fertile offspring a tendency to 
produce sterile members with the same modifications. This pro- 
cess must have been repeated many times, until that prodigious 
amount of difierence between the fertile and sterile females of the 
same species has been produced, which we see in many .social 

But we have not as yet touched on the acme ot the difficulty ; 
namely, the fact that the neuters of several ants differ, not only 
from the fertile females and males, but from each other, sometimes 
to an almost incredible degree, and are thus divided into two or even 
three castes. The castes, moreover, do not commonly graduate 
into each other, but are perfectly well defined ; being as distinct 
Irom each other as are any two species of the same genus, or rather 
as any two genera of the same family. Thus in Eciton, there are 
working and soldier neuters, with jaws and instincts extraordinarily 


different : in Cryptocerus, the workers of one caste alone carry a 
wonderful sort of shield on their heads, the use of which is quite 
unknown: in the Mexican Myrmecocystus, the workers of one 
caste never leave the nest ; they are fed by the workers of another 
caste, and they have an enormously developed abdomen which 
secretes a sort of honey, supplying the place of that excreted by the 
aphides, or the do mestic ca ttle as they may be called, which our 
EuropeaiTahTs gu ard and impri son. 

It will indeed be thought that I have an overweening confidence 
in the principle of natural selection, when I do not admit that 
such wonderful and well-established facts at once annihilate the 
theory. In the simpler case of neuter insects all of one caste, 
which, as I believe, have been rendered difrerent from the fertile 
males and females through natural selection, we may conclude from 
the analogy of ordinary variations, that the successive, slight, pro- 
fitable modifications did not first arise in all the neuters in the same 
nest, but in some few alone; and that by the survival of the 
conmiunities with females which produced most neuters having 
the advantageous modification, all the neuters ultimately came to 
be thus characterised. According to this view we ought occasion- 
ally to find in the same nest neuter insects, presenting gradations 
of structure ; and this we do find, even not rarely, considering how 
few neuter insects out of Europe have been carefully examined. 
Mr. F. Smith has shown that the neuters of several British ants 
differ surprisingly from each other in size and sometimes in colour ; 
and that the extreme forms can be linked together by individuals 
taken out of the same nest: I have myself compared perfect 
gradations of this kind. It sometimes happens that the larger or 
the smaller sized workers are the most numerous ; or that both 
large and small are numerous, whilst those of an intermediate size 
are scanty in numbers. Formica fiava has larger and smaller 
workers, with some few of intermediate size ; and, in this species, 
as Mr. F. Smith has observed, the larger workers have simple eyes 
(ocelli), which though small can be plainly distinguished, whereas 
the smaller workers have their ocelli rudimentary. Having 
carefully dissected several specimens of these workers, I can affirm 
that the eyes are far more rudimentary in the smaller workers than 
can be accounted for merely by their proportionally lesser size ; and 
I fully believe, though I dare not assert so positively, that the 
workers of intermediate size have their ocelli in an exactly inter- 
mediate condition. So that here we have two bodies of sterile 
workers in the same nest, differing not only in size, but in their 
organs of vision, yst connected by some f3w members in an inter- 


mediate condition. I may diiiress by adding, that if the smaller 
workers had been the most useful to the community, and those 
males and females had been continually selected, which produced 
more and more of the smaller workers, until all the workers were 
in this condition ; we should then have had a species of ant with 
neuters in nearly the same condition as those of Myrmica. For 
the workers of Myrmica have not even rudiments of ocelli, thougli 
the male and female ants of this genus have well-developed ocelli. 

I may give one other case : so confidently did I expect occasion- 
ally to find gradations of important structures between the different 
castes of neuters in the same species, that I gladly availed myself 
of Mr. F. Smith's offer of numerous specimens from the same 
nest of the driver ant (Anomma) of West Africa. The reader will 
perhaps best appreciate the amount of difierence in these workers, 
iDy my giving not the actual measurements, but a strictly accurate 
illustration : the difference was the same as if we were to see a set 
cf workmen building a house, of whom many were five feet four 
inches high, and many sixteen feet high ; but we must in addition 
auppose that the larger workmen had heads four instead of three 
times as big as those of the smaller men, and jaws nearly fivo 
times as big. The jaws, moreover, of the working ants of the 
several sizes differed wonderfully in shape, and in the form and 
number of the teeth. But the important fact for us is, that, 
though the workers can be grouped into castes of different sizes, 
yet they graduate insensibly into each other, as does the widely- 
different structure of their jaws. I speak confidently on this 
latter point, as Sir J. Lubbock made drawings for me, with the 
camera lucida, of the jaws which I dissected from the workers of 
the several sizes. Mr. Bates, in his interesting * Naturalist on the 
Amazons,' has described analogous cases. 

AVith these facts before me, I believe that natural selection, by 
acting on the fertile ants or parents, could form a species which 
should regularly produce neuters, all of large size with one form 
of jaw, or all of small size with widely different jaws ; or lastly, 
and this is the greatest difSculty, one set of workers of one size 
and structure, and simultaneously another set of workers of a dif- 
ferent size and structure ; — a graduated series having first been 
formed, as in the case of the driver ant, and then the extreme 
forms having been produced in greater and greater numbers, through 
the survival of the parents which generated them, until none with 
an intermediate structure were produced. 

An analogous explanation has been given by Mr. Wallace, of 
the equally complex case, of certain Malayan Butterflies regularly 

Chap. VIIL] SUMMARY. 233 

appearing under two or even three distinct female foiins ; and by 
Fritz Miiller, of certain Brazilian crustaceans likewise appearing 
iinder two widely distinct male forms. But this subject need not 
here be discussed. 

I have now explained how, as I believe, the wonderful fact of 
two distinctly defined castes of sterile workers existing in the 
same nest, both widely different from each other and from their 
parents, has originated. We can see how useful their production 
may have been to a social community of ants, on the same principle 
that the division of labour is useful to civilised man. Ants, how- 
ever, work by inherited instincts and by inherited organs or tools, 
whilst man works by acquired knowledge and manufactured instru- 
ments. But I must confess, that, with all my faith in natural 
selection, I should never have anticipated that this principle could 
have been efficient in so high a degree, had not the case of these neuter 
insects led me to this conclusion. I have, therefore, discussed this 
case, at some little but wholly insufficient length, in order to show 
the power of natural selection, and likewise because this is by far the 
most serious special difficulty which my theory has encountered. 
The case, also, is very interesting, as it proves that with animals, as 
with plants, any amount of modification may be effected by the 
accumulation of numerous, slight, spontaneous variations, which 
are in any way profitable, without exercise or habit having been 
brought into play. For peculiar habits confined to the workers or 
sterile females, however long they might be followed, could not 
possibly affect the males and fertile females, which alone leave 
descendants. I am surprised that no one has hitherto advanced 
this demonstrative case of neuter insects, against the well-known 
doctrine of inherited habit, as advanced by Lamarck. 


I have endeavoured in this chapter briefly to show that the 
menial qualities of our jlompi^in animals vary, and that the varia- 
tions are inherited. Still more briefly I have attempted to show 
that instincts vary slightly in a state of nature . No one will dis- 
pute that instincts are of the highest importance to each animah 
Therefore there is no real difficulty^ uMeTcTiangmg~cohditions ot life, 
in natural selection accumulating to any extent slight modifications 
of instinct which are in any way useful. In many cases habit or 
use and disuse have probably come into play. I do not pretend 
tEat the facts given in this chapter strengthen in any great degree 
my tleory ; but none of tho cases ot difficulty, to the best of my 
judgment, annihilate it. On the other hand, the fact that instincts 

234 SUMMARY. [Chap. VIIL 

are not always absolutely perfect and are liable to mistakes : — tliat 
no instinct can be shown to have been produced for tho good of 
other animals, though animals take advantag e of the i nstincts of 
others ; — that the canon in natural "History, of "Natura non facit 
saltum," is applicable to instincts as well as to corporeal structure, 
and is plainly explicable on the foregoing views, but is other- 
wise inexplicable, — all tend to corroborate the theory of natural 

This theory is also strengthened by some few other facts in 
regard to instincts ; a-s by that common case of closely allied, but 
distinct, species, when inhabiting distant parts of tbe world and 
living under considerably different conditions of life, yet often 
retaining nearly the same instincts. For instance, we can under- 
stand, on the principle of inheritance, bow it is that th.e thrush of 
tropical South America lines its nest with mud, in the same peculiar 
manner as does our British thrush ; how it is that the Hornbills of 
Africa and India have the same extraordinary instinctof plastering, 
up and imprisoning the females in a hole in a tree, with only a 
small hole left in the plaster through which the males feed them 
and their young when hatched ; how it is that the male wrens 
(Troglodytes) of North America build "cock-nests," to roost in, 
like the males of our Kitty- wrens, — a habit wholly unlike that of 
any other known bird. Finally, it may not be a logical deduction, 
but to my imagination it is far more satisfactory to look at such 
instincts as the young cuckoo ejecting its foster-brothers, — ants 
making slaves, — the larvas of ichneumonidse feeding within the 
live bodies of caterpillars, — not as specially endowed or created 
instincts, but as small consequences of one general law leading to 
the advancement of all organic beings, — namely, multiply, vary, 
let the strongest live and the weakest die. 

Chap. IX.] HYBRIDISM. 235 



Difitinction between the sterility of first crosses and of hybrids — Sterility 
various in degree, not universal, affected by close interbreeding, re- 
moved by domestication — Laws governing the sterility of hybrids — 
Sterility not a special endowment, but incidental on other differences, 
not accumulated by natural selection — Causes of the sterility of first 
crosses and of hybrids — Parallelism between the effects of changed 
conditions of life and of crossing — Dimorphism and trimorphism — 
Fertility of varieties when crossed and of their mongrel offspring not 
universal — Hybrids and mongrels compared independently of their 
fertility — Summary. 

The view commonly entertained by naturalists is that species, when 
intercrossed, have been specially endowed with, sterility, in order to 
prevent their confusion. This view certainly seems at first highly 
probable, for species living together could hardly have been kept 
distinct had they been capable of freely crossing. The subject is 
in many ways important for us, more especially as the sterility of 
species when first crossed, and that of their hybrid offspring, cannot 
have been acquired, as I shall show, by the preservation of suc- 
cessive profitable degrees of sterility. It is an incidental result of 
differences in the reproductive systems of the parent-species. 

In treating this subject, two classes of facts, to a large extent 
fundamentally different, have generally been confounded ; namely, 
the steriUty of species when first crossed, and the sterility of the 
liybrids produced from them. 

Pure species have of course their organs of reproduction in a per- 
fect condition, yet when intercrossed they produce either few or no 
offspring. Hybrids, on the other hand, have their reproductive 
organs functionally impotent, as may be clearly seen in the state of 
the male element in both plants and animals ; though the formative 
organs themselves are perfect in structure, as far as the microscope 
reveals. In the first case the two sexual elements which go to form 
the embryo are perfect ; in the second case they are either not at all 
developed, or are imperfectly developed. This distinction is im- 
portant, when the cause of the sterility, which is common to the 

236 HYBRIDISM. [Chap. IX 

two cases, has to be considered, TLe distinction probably has been 
slurred over, owing to the sterility in both cases being looked on as 
a special endowment, beyond the province of our reasoning powers. 

The fertility of varieties, that is of the forms known or believed 
to be descended from common parents, when crossed, and likewise 
the fertility of their mongrel offspring, is, with reference to my 
theory, of equal importance with the sterility of species ; for it 
seems to make a broad and clear distinction between varieties and 

Degrees of Sterility. — First, for the sterility of species when 
crossed and of their hybrid offspring. It is impossible to study the 
several memoirs and works of those two conscientious and admirable 
observers, Kolreuter and Gartner, who almost devoted their lives to 
this subject, without being deeply impressed with the high gene- 
rality of some degree of sterility. Koheuter makes the rule 
universal ; but then he cuts the knot, for in ten cases in which he 
found two forms, considered by most authors as distinct species, 
quite fertile together, he unhesitatingly ranks them as varieties. 
Gartner, also, makes the rule equally universal ; and he disputes 
the entire fertility of Kolreuter' s ten cases. But in these and in 
many other cases, Gartner is obliged carefully to count the seeds, 
in order to show that there is anj^ degree of sterility. He always 
compares the maximum number of seeds produced by two species 
when first crossed, and the maximum produced by their hybrid 
offspring, with the average number produced by both pure parent- 
species in a state of nature. But causes of serious error here inter- 
vene : a plant, to be hybridised, must be castrated, and, what is 
often more important, must be secluded in order to prevent pollen 
being brought to it by insects from other plants. Nearly all the 
plants experimented on by Gartner were potted, and were kept in a 
chamber in his house. That these processes are often injurious to 
the fertility of a plant cannot be doubted ; for Gartner gives in his 
table about a score of cases of plants which he castrated, and 
artificially fertilised with their own pollen, and (excluding all cases 
such as the Leguminosse, in which there is an acknowledged diffi- 
culty in the manipulation) half of these twenty plants had their 
fertility in some degree impaired. Moreover, as Gartner repeatedly 
crossed some forms, such as the common red and blue pimpernels 
(Anagallis arvensis and coerulea), which the best botanists rank as 
varieties, and found them absolutely sterile, we may doubt whether 
many species are really so sterile, when intercrossed, as he believed. 

It is certain, on the one hand, that the sterility of various speciea 
when crossed is so different in degree and graduates away so in- 


sensibly, and, on the other hand, that the fertility of pure species 
is so easily affected by various circumstances, that for all practical 
purposes it is most difficult to say where perfect fertility ends and 
sterility begins. I think no better evidence of this can be required 
than that the two most experienced observers who have ever lived, 
namely Kolreuter and Gartner, arrived at diametrically opposite 
conclusions in regard to some of the very same forms. It is also 
most instructive to compare — but I have not space here to enter on 
details — the evidence advanced by our best botanists on the question 
whether certain doubtful forms should be ranked as species oi 
varieties, with the evidence from fertility adduced by different 
hybridisers, or by the same observer from experiments made during 
different years. It can thus be shown that neither sterility nor 
fertility affords any certain distinction between species and varieties. 
The evidence from this source graduates away, and is doubtful in 
the same degree as is the evidence derived from other constitutional 
and structural differences. 

In regard to the sterility of hybrids in successive generations ; 
though Gartner was enabled to rear some hybrids, carefully guard- 
ing them from a cross with either pure parent, for six or seven, and 
in one case for ten generations, yet he asserts positively that their 
fertility never increases, but generally decreases greatly and sud- 
denly. With respect to this decrease, it may first be noticed that 
when any deviation in structure or constitution is common to both 
parents, this is often transmitted in an augmented degree to the 
offspring ; and both sexual elements in hybrid plants are already 
affected in some degree. But I believe that their fertility has been 
diminished in nearly all these cases by an independent cause, 
namely, by too close interbreeding. I have made so many experi- 
ments and collected so many facts, showing on the one hand that 
an occasional cross with a distinct individual or variety increases 
the vigour and fertility of the offspring, and on the other hand that 
very close interbreeding lessens their vigour and fertility, that I 
cannot doubt the correctness of this conclusion. Hybrids are seldom 
raised by experimentalists in great numbers; and as the parent- 
species, or other allied hybrids, generally grow in the same garden, 
the visits of insects muat be carefully prevented during the flowering 
season: hence hybrids, if left to themselves, will generally be 
fertilised during each generation by pollen from the same flower ; 
and this would probably be injurious to their fertility, already 
lessened by their hybrid origin. I am strengthened in this con- 
viction by a remarkable statement repeatedly made by Gartner, 
namely, that if even the less fertile hybrids be artificially fertilisal 

238 HYBRIDISM. [Chap. IX, 

with hybrid pollen of the same kind, their fertility, notwithstanding 
the frequent ill effects from manipulation, sometimes decidedly 
increases, and goes on increasing. Now, in the process of artificial 
fertilisation, pollen is as often taken by chance (as I know from my 
own experience) from the anthers of another flower, as from the 
anthers of the flower itself which is to be fertilised ; so that a cross 
between two flowers, though probably often on the same plant, 
would be thus effected. Moreover, whenever complicated experi- 
ments are in progress, so careful an observer as Gartner would have 
castrated his hybrids, and this would have ensured in each genera- 
tion a cross with pollen from a distinct flower, either from the same 
plant or from another plant of the same hybrid nature. And thus, 
the strange fact of an increase of fertility in the successive genera- 
tions of artificially fertilised hybrids, in contrast with thoso 
spontaneously self-fertilised, may, as I believe, be accounted for by 
too close interbreeding having been avoided. 

Now let us turn to the results arrived at by a third most experi- 
enced hybridiser, namely, the Hon. and Kev. W. Herbert. He is 
as emphatic in his conclusion that some hybrids are perfectly fertile 
— as fertile as the pure parent-species — as are Kolreuter and Gartner 
that some degree of sterility between distinct species is a universal 
law of nature. He experimented on some of the very same species 
as did Gartner. The difference in their results may, I think, be in 
part accounted for by Herbert's great horticultural skill, and by his 
having hot-houses at his command. Of his many important state- 
ments I will here give only a single one as an example, namely, 
that " every ovule in a pod of Crinum capense fertilized by 0. 
revolutum produced a plant, which I never saw to occur in a case 
of its natural fecundation." So that here we have perfect or even 
more than commonly perfect fertility, in a first cross between two 
distinct species. 

This case of the Crinum leads me to refer to a singular fact, 
namely, that individual plants of certain species of Lobelia, Yer- 
bascum and Passiflora, can easily be fertilised by pollen from a 
distinct species, but not by pollen from the same plant, though this 
pollen can be proved to be perfectly sound by fertilising other plants 
or species. In the genus Hippeastrum, in Corydalis as shown by 
Professor Hildebrand, in various orchids as shown by Mr. Scott and 
Fritz Miiller, all the individuals are in this peculiar condition. So 
that with some species, certain abnormal individuals, and in other 
species all the individuals, can actually be hybridised much more 
readily than they can be fertilised by pollen from the same individual 
plant ! To gire one instance, a bulb of Hippeastrum aulicum pro- 


duced four flowers ; three were fertilised by Herbert with their own 
pollen, and the fourth was subsequently fertilised by the pollen of 
a compound hybrid descended from three distinct species : the 
result was that " the ovaries of the three first flowers soon ceased to 
"grow, and after a few days perished entirely, whereas the pod 
" impregnated by the pollen of the hybrid made vigorous growth 
" and rapid progress to maturity, and bore good seed, which vege- 
" tated freely." Mr. Herbert tried similar experiments during many 
years, and always with the same result. These cases serve to show 
on what slight and mysterious causes the lesser or greater fertility 
of a species sometimes depends. 

The practical experiments of horticulturists, though not made 
with scientific precision, deserve some notice. It is notorious in 
how complicated a manner the species of Pelargonium, Fuchsia, 
Calceolaria, Petunia, Khododendron, &c«, have been crossed, yet 
many of these hybrids seed freely. For instance, Herbert asserts 
that a hybrid from Calceolaria integrifolia and plantaginea, species 
most widely dissimilar in general habit, " reproduces itself as per- 
fectly as if it had been a natural species from the mountains of 
Chili." 1 have taken some pains to ascertain the degree of fertility 
of some of the complex crosses of Khododendrons, and I am assured 
that many of them are perfectly fertile. Mr. C. Noble, for instance, 
informs me that he raises stocks for grafting from a hybrid between 
Rhod. ponticum and catawbiense, and that this hybrid '* seeds as 
freely as it is possible to imagine." Had hybrids, when fairly 
treated, always gone on decreasing in fertility in each successive 
generation, as Gartner believed to be the case, the fact would have 
been notorious to nursery-men. Horticulturists raise large beds of 
the same hybrid, and such alone are fairly treated, for by insect 
agency the several individuals are allowed to cross freely with each 
other, and the injurious influence of close interbreeding is thus 
prevented. Any one may readily convince himself of the efficiency 
of insect-agency by examining the flowers of the more sterile kinds 
of hybrid Khododendrons, which produce no pollen, for he will 
find on their stigmas plenty of pollen brought from other flowers. 

In regard to animals, much fewer experiments have been care- 
fully tried than with plants. If our systematic arrangements can 
be trusted, that is, if the genera of animals are as distinct from each 
other as are the genera of plants, then we may infer that animals 
more widely distinct in the scale of nature can be crossed more 
easily than in the case of plants ; but the hybrids themselves are, 
I think, more sterile. It should, however, be borne in mind that, 
owing to few animals breeding freely under confinemsnt, few 

240 HYBRIDISM [Chap. IX. 

experiments have been fairly tried : for instance, the canary-hird 
has been crossed with nine distinct species of finches, but, as tot 
one of these breeds freely in confinement, we have no right to 
expect that the first crosses between them and the canary, or that 
their hybrids, should be perfectly fertile. Again, with respect to 
the fertility in successive generations of the more fertile hybrid 
animals, I hardly know of an instance in which two families of the 
same hybrid have been raised at the same time from different 
parents, so as to avoid the ill effects of close interbreeding. On the 
contrary, brothers and sisters have usually been crossed in each 
successive generation, in opposition to the constantly repeated 
admonition of every breeder. And in this case, it is not at all 
surprising that the inherent sterility in the hybrids should have 
gone on increasing. 

Although I know of hardly any thoroughly well-authenticated 
cases of perfectly fertile hybrid animals, I have reason to believe 
that the hybrids from Cervulus vaginalis and Eeevesii, and from 
Phasianus colchicus with P. torquatus, are perfectly fertile. M. 
Quatrefages states that the hybrids from two moths (Bombyx 
cynthia and arrindia) were proved in Paris to be fertile inter se 
for eight generations. It has lately been asserted that two such 
distinct species as the hare and rabbit, when they can be got to 
breed together, produce offspring, which are highly fertile when 
crossed with one of the parent-species. The hybrids from the 
common and Chinese geese (A. cygnoides), species which are so 
different that they are generally ranked in distinct genera, have 
often bred in this country with either pure parent, and in one 
single instance they have bred inter se. This was effected by 
Mr. Eyton, who raised two hybrids from the same parents, but 
from different hatches ; and from these two birds he raised no less 
than eight hybrids (grandchildren of the pure geese) from one nest. 
In India, however, these cross-bred geese must be far more fertile ; 
for I am assured by two eminently capable judges, namely Mr. 
Blyth and Capt. Button, that whole flocks of these crossed geese 
are kept in various parts of the country ; and as they are kept for 
profit, where neither pure parent-species exists, they must certainly 
be highly or perfectly fertile. 

With our domesticated animals, the various races when crossed 
together are quite fertile ; yet in many cases they are descended 
from two or more wild species. From this fact we must conclude 
either that the aboriginal parent-species at first produced perfectly 
fertile hybrids, or that the hybrids subsequently reared under 
domestication became quite fertile. This latter alternative, which 


was first propounded by Pallas, seems by far the most probable, and 
can, indeed, hardly be doubted. It is, for instance, almost certain 
that our dogs are descended from several wild stocks ; yet, with 
perhaps the exception of certain indigenous domestic dogs of South 
America, all are quite fertile together; but analogy makes me 
greatly doubt, whether the several aboriginal species would at first 
have freely bred together and have i)roduced quite fertile hybrids. 
So again I have lately acquired decisive evidence that the 
crossed offspring from the Indian humped and common cattle are 
inter se perfectly fertile ; and from the observations by Kiitimeyer 
on their important osteological differences, as well as from those 
by Mr. Blyth on their differences in habits, voice, constitution, &c., 
these two forms must be regarded as good and distinct species. 
The same remarks may be extended to the two chief races of 
the pig. We must, therefore, either give up the belief of the 
universal sterility of species when crossed; or we must look at 
this sterility in animals, not as an indelible characteristic, but 
as one capable of being removed by domestication. 

Finally, considering all the ascertained facts on the intercross- 
ing of plants and animals, it may be concluded that some degree 
of sterility, both in first crosses and in hybrids, is an extremely 
general result ; but that it cannot, under our present state of 
knowledge, be considered as absolutely universal. 

Laws governing the Sterility of first Crosses and of Hybrids. 

We will now consider a little more in detail the laws governing 
the sterility of first crosses and of hybrids. Our chief object will 
be to see whether or not these laws indicate that species have 
been specially endowed with this quality, in order to prevent 
their- crossing and blending together in utter confusion. The fol- 
lowing conclusions are drawn up chiefly from Gartner's admirable 
work on the hybridisation of plants. I have taken much pains 
to ascertain how far they apply to animals, and, considering how 
scanty our knowledge is in regard to hybrid animals, I have 
been surprised to find how generally the same rules apply to both 

It has been already remarked, that the degree of fertility, both 
of first crosses and of hybrids, graduates from zero to perfect 
fertility. It is surprising in how many curious ways this grada- 
tion can be shown ; but only the barest outline of the facts can 
here be given. When pollen from a plant of one family is placed 
on the stigma of a plant of a distinct family, it exerts no mere 
influence than so much inorgaric dust. From this absolute zero of 


fertility, the pollen of different species applied to tlie stigma of 
some one species of tli3 same genus, yields a perfect gradation in 
tlae number of seeds produced, up to nearly complete or even quite 
complete fertility ; and, as we have seen, in certain abnormal caseSj 
even to an excess of fertility, beyond that which the plant's own 
pollen produces. So in hybrids themselves, there are some which 
never have produced, and probably never would produce, even with 
the pollen of the pure parents, a single fertile seed : but in some of 
these cases a first trace of fertility may be detected, by the pollen 
of one of the pure parent-species causing the flower of the hybrid to 
wither earlier than it otherwise would have done ; and the early 
withering of the flower is well known to be a sign of incipient 
fertilisation. From this extreme degree of sterility we have self- 
fertilised hybrids producing a greater and greater number of seeds 
up to perfect fertility. 

The hybrids raised from two species which are very difficult 
to cross, and which rarely produce any offspring, are generally very 
sterile ; but the parallelism between the difficulty of making a first 
cross, and the sterility of the hybrids thus produced — two cl'asses of 
facts which are generally confounded together — is by no means 
strict. There are many cases, in which two pure species, as in the 
genus Yerbascum, can be united with unusual facility, and produce 
numerous hybrid-offspring, yet these hybrids are remarkably sterile. 
On the other hand, there are siDccies which can be crossed very 
rarely, or with extreme difficulty, but the hybrids, when at last 
produced, are very fertile. Even within the limits of the same 
genus, for instance in Dianthus, these two OjDposite cases occur. 

The fertility, both of first crosses and of hybrids, is more easily 
affected by unfavourable conditions, than is that of pure species. 
But the fertility of first crosses is likewise innately variable ; for 
it is not always the same in degree when the same two species are 
crossed under the same circumstances ; it depends in part upon the 
constitution of the individuals which happen to have been chosen 
for the experiment. So it is with hybrids, for their degree of 
fertility is often found to differ greatly in the several individuals 
raised from seed out of the same capsule and exposed to the same 

By the term systematic affinity is meant, the genera] resemblance 
between species in structure and constitution. Now the fertility of 
first crosses, and of the hybrids produced from them, is largely 
governed by their systematic affinity. This is clearly shown by 
hybrids never having been raised between species ranked by sys- 
tematists in distinct families j and on the other hand, by very 


closely allied species generally uniting with facility. But the 
correspondence between systematic affinity and the facility of 
crossing is by no means strict. A multitude of cases could be given 
of very closely allied species which will not unite, or only with 
extreme difficulty ; and on the other hand of very distinct species 
which unite with the utmost facility. In the same family there 
may be a genus, as Dianthus, in which very many species can most 
readily be crossed ; and another genus, as Silene, in which the most 
persevering efforts have failed to produce between extremely close 
species a single hybrid. Even within the limits of the same genus, 
we meet with this same difference ; for instance, the many species ol 
Nicotiana have been more largely crossed than the species of almost 
any other genus ; but Gartner found that N. acuminata, which is 
not a particularly distinct species, obstinately failed to fertilise, or 
to be fertilised by no less than eight other species of Nicotiana. 
Many analogous facts could be given. 

No one has been able to point out what kind or what amount of 
difference, in any recognisable character, is sufficient to prevent two 
species crossing. It can be shown that plants most widely different in 
habit and general appearance, and having strongly marked differ- 
ences in every part of the flower, even in the pollen, in the fruit, 
and in the cotyledons, can be crossed. Annual and perennial plants, 
deciduous and evergreen trees, plants inhabiting different stations 
and fitted for extremely different climates, can often be crossed 
with ease. 

By a reciprocal cross between two species, I mean the case, 
for instance, of a female-ass being first crossed by a stallion, and 
then a mare by a male-ass : these two species may then be said 
to have been reciprocally crossed. There is often the widest 
possible difference in the facility of making reciprocal crosses. 
Such cases are highly important, for they prove that the capacity 
in any two species to cross is often completely independent of their 
systematic affinity, that is of any difference in their structure 
or constitution, excepting in their reproductive systems. The 
diversity of the result in reciprocal crosses between the same two 
species was long ago observed by Kolreuter. To give an instance : 
Mirabilis jalapa can easily be fertilised by the pollen of M. longi- 
flora, and the hybrids thus x^roduced are sufficiently fertile ; but 
Kolreuter tried more than two hundred times, during eight fol- 
lowing years, to fertilise reciprocally M. longiflora with the pollen of 
M. jalapa, and utterly failed. Several other equally striking cases 
could be given. Thuret has observed the same fact with certain 
sea- weeds or Fuci. Gartner, moreover, found that this difference of 

K 2 


facility in making reciprocal crosses is extremely common in a 
lesser degree. He has observed it even between closely related 
forms (as Matthiola annua and gJabra) which many botanists rank 
only as varieties. It is also a remarkable fact, that hybrids raised 
from reciprocal crosses, though of course compounded of the very 
same two species, the one species having first been used as the 
father and then as the mother, though they rarely differ in external 
characters, yet generally differ in fertility in a small, and occa- 
sionally in a high degree. 

Several other singular rules could be given from Gartner : foT 
instance, some species have a remarkable power of crossing with 
other species ; other species of the same genus have a remarkable 
power of impressing their likeness on their hj'^brid offspring ; but 
these two powers do not at all necessarily go together. There are 
certain hybrids which, instead of having, as is usual, an intermediate 
character between their two parents, always closely resemble one of 
them ; and such hybrids, though externally so like one of their pure 
parent-species, are with rare exceptions extremely sterile. So again 
amongst hybrids which are usually intermediate in structure 
between their parents, exceptional and abnormal individuals some- 
times are bom, which closely resemble one of theii pure parents ; 
and these hybrids are almost always utterly sterile, even when the 
other hybrids raised from seed from the same capsule have a con- 
iiiderable degree of fertility. These facts show how completely the 
fertility of a hybrid may be independent of its external resemblance 
to either pure parent. 

Considering the several rules now given, which govern the 
fertility of first crosses and of hybrids, we see that when forms, 
which must be considered as good and distinct species, are united, 
their fertility graduates from zero to perfect fertility, or even to 
fertility under certain conditions in excess; that their fertility, 
besides being eminently susceptible to favourable and unfavourable 
conditions, is innately variable ; that it is by no means always 
the same in degree in the first cross and in the hybrids produced 
from this cross ; that the fertility of hybrids is not related to the 
degree in which they resemble in external appearance either parent ; 
and lastly, that the facility of making a first cross between any 
two species is not always governed by their systematic affinity or 
degree of resemblance to each other. This latter statement is 
clearly proved by the difference in the result of reciprocal cresses 
between the same two species, for, according as the one species 
or the other is used as the father or the mother, there is generally 
Bome diffefence, and occasionally the widest possible difference^ 


ia the facility of effecting an union. The hybrids, moreover, 
produced from reciprocal crosses often differ in fertility. 

Now do these complex and singular rules indicate that species 
have been endowed with sterility simply to prevent their becoming 
confounded in nature ? I think not. For why should the sterility 
be so extremely different in degree, when various species are crossedj 
all of which we must suppose it would be equally important to keep 
from blending together? Why should the degree of sterility be 
innately variable in the individuals of the same species? Why 
should some species cross with facility, and yet produce very sterile 
hybrids ; and other species cross with extreme difficulty, and yet 
produce fairly fertile hybrids? Why should there often be so 
great a difference in the result of a reciprocal cross between the 
same two species ? Why, it may even be asked, has the production 
of hybrids been permitted ? To grant to species the special power 
of producing hybrids, and then to stop their further propagation by 
different degrees of sterility, not strictly related to the facility of 
the first union between their parents, seems a strange arrangement. 

The foregoing rules and facts, on the other hand, appear to me 
clearly to indicate that the sterility both of first crosses and of 
hybrids is simply incidental or dependent on unknown differences 
in their reproductive systems ; the differences being of so peculiar 
and limited a nature, that, in reciprocal crosses between the same 
two species, the male sexual element of the one will often freely act 
on the female sexual element of the other, but not in a reversed 
direction. It will be advisable to explain a little more fully by 
an example what I mean by sterility being incidental on other 
differences, and not a specially endowed quality. As th-e capacity 
of one plant to be grafted or budded on another is unimportant for 
their welfare in a state of nature, I presume that no one will suppose 
that this capacity is a specially endowed quality, but will admit that 
it is incidental on differences in the laws of growth of the two 
plants. We can sometimes see the reason why one tree will not 
take on another, from differences in their rate of growth, in the 
hardness of their wood, in the period of the flow or nature of their 
sap, &c. ; but in a multitude of cases we can assign no reason what- 
ever. Great diversity in the size of two plants, one being woody 
and the other herbaceous, one being evergreen and the other deci- 
duous, and adaptation to widely different climates, do not always 
prevent the two grafting together. As in hybridisation, so with 
grafting, the capacity is limited by systematic affinity, for no one 
has been able to graft together trees belonging to quite distinct 
families ; and, on the other hand, closely allied species, and varieties 


of the same species, can usually, but not invariably, be grafted -with 
ease. But this capacity, as in hybridisation, is by no means abso- 
lul^ly governed by systematic afiSnity. Although many distinct 
genera within the same family have been grafted together, in other 
cases species of the same genus will not take on each other. The 
pear can be grafted far more readily on the quince, which is ranked 
as a distinct genus, than on the apple, which is a member of the 
same genus. Even different varieties of the pear take with different 
degrees of facility on the quince ; so do different varieties of the 
apricot and peach on certain varieties of the plum. 

As Gartner found that there was sometimes an innate difference 
in different individuals of the same two species in crossing ; so 
Sageret believes this to be the case with different individuals of 
the same two species in being grafted together. As in reciprocal 
crosses, the facility of effecting an union is often very far from 
equal, so it sometimes is in grafting ; the common gooseberry, for 
instance, cannot be grafted on the currant, whereas the currant will 
take, though with difficulty, on the gooseberry. 

We have seen that the sterility of hybrids, which have their 
reproductive organs in an imperfect condition, is a different case 
from the difficulty of uniting two pure species, which have their 
reproductive organs perfect ; yet these two distinct classes of 
cases run to a large extent parallel. Something analogous occurs 
in grafting ; for Thoum found that three species of Eobinia, 
which seeded freely on their own roots, and which could be 
grafted with no great difficulty on a fourth species, when thus 
grafted were rendered barren. On the other hand, certain species of 
Sorbus, when grafted on other species yielded twice as much fruit 
as when on their own roots. We are reminded by this latter fact of 
the extraordinary cases of Hippeastrum. Passi flora, &c., which seed 
much more freely when fertilised with the pollen of a distinct 
species, than when fertilised with pollen from the same plant. 

We thus see, that, although there is a clear and great difference 
between the mere adhesion of grafted stocks, and the union of 
the male and female elements in the act of reproduction, yet that 
there is a rude degree of parallelism in the results of grafting and of 
crossing distinct species. And as we must look at the curious 
and complex laws governing the facility with which trees can be 
grafted on each other as incidental on unknown differences in their 
vegetative systems, so I believe that the still more complex laws 
governing the facility of first crosses are incidental on unknown 
differences in their reproductive systems. These differences in both 
cases, follov; to a certain extent, as might have been expected* 


systematic affinity, by which term every kind of resemblance and 
dissimilarity between organic beings is attempted to be expressed. 
The facts by no means seem to indicate that the greater or lesser 
difficulty of either grafting or crossing various species has been a 
special endowment ; although in the case of crossing, the difficulty 
is as important for the endurance and stability of specific forma, 
as in the case of grafting it is unimportant for their welfare. 

Origin and Causes of the Sterility of first Crosses and of 


At one time it appeared to me probable, as it has to others, that 
the sterility of first crosses and of hybrids might have been slowly 
acquired through the natural selection of slightly lessened degrees 
of fertility^ which, like any other variation, spontaneously appeared 
in certain individuals of one variety when crossed with those of 
another variety. For it would clearly be advantageous to two 
varieties or incipient species, if they could be kept from blending, 
on the same principle that, when man is selecting at the same time 
two varieties, it is necessary that he should keep them separate. 
In the first place, it may be remarked that species inhabiting 
distinct regions are often sterile when crossed ; now it could clearly 
have been of no advantage to such separated species to have been 
rendered mutually sterile, and consequently this could not have 
been effected through natural selection ; but it may perhaps be 
argued, that, if a species was rendered sterile with some one 
compatriot, sterility with other species would follow as a necessary 
contingency. In the second place, it is almost as much opposed to 
the theory of natural selection as to that of special creation, that in 
reciprocal crosses the male element of one form should have been 
rendered utterly impotent on a second form, whilst at the same 
time the male element of this second form is enabled freely to 
fertilise the first form ; for this peculiar state of the reproductive 
system could hardly have been advantageous to either species. 

In considering the probability of natural selection having come 
into action, in rendering species mutually sterile, the greatest 
difficulty will be found to lie in the existence of many graduated 
steps from slightly lessened fertility to absolute sterility. It may 
be admitted that it would profit an incipient species, if it were 
rendered in some slight degree sterile when crossed with its parent 
form or w^ith some other variety ; for thus fewer bastardised and 
deteriorated offspring would be produced to commingle their blood 
with the new species in process of formation. But he who will take 
the trouble to reflect on the steps by which this first degree of 


sterility could be increased tlirougli natural selection to that high 
degree which is common with so many species, and which is 
universal with species which have been differentiated to a generic 
or family rank, will find the subject extraordinarily complex. 
After mature reflection it seems to me that this could not have been 
effected through natural selection. Take the case of any two 
species which, when crossed, produce few and sterile offspring ; now, 
what is there which could favour the survival of those individuals 
which happened to be endowed in a slightly higher degree with 
mutual infertility, and which thus approached by one small step 
towards absolute sterility ? Yet an advance of this kind, if the 
theory of natural selection be brought to bear, must have incessantly 
occurred with many species, for a multitude are mutually quite 
barren. With sterile neuter insects we have reason to believe that 
modifications in their structure and fertility have been slowly 
accumulated by natural selection, from an advantage having been 
thus indirectly given to the community to which they belonged 
over other communities of the same species ; but an individual 
animal not belonging to a social community, if rendered slightly 
sterile when crossed with some other variety, would not thus itself 
gain any advantage or indirectly give any advantage to the other 
iadividuals of the same variety, thus leading to their preservation. 

But it would be superfluous to discuss this question in detail; 
for with plants we have conclusive evidence that the sterility of 
crossed species must be due to some principle, quite independent of 
natural selection. Both Gartner and Kolreuter have proved that in 
genera including numerous species, a series can be formed from 
species which when crossed yield fewer and fewer seeds, to species 
which never produce a single seed, but yet are affected by the 
pollen of certain other species, for the germen swells. It is here 
manifestly impossible to select the more sterile individuals, which 
have already ceased to yield seeds ; so that this acme of sterility, 
when the germen alone is affected, cannot have been gained through 
selection ; and from the laws governing the various grades of sterility 
being so uniform throughout the animal and vegetable kingdoms, 
we may infer that the cause, whatever it may be, is the same or 
nearly the same in all cases. 

We will now look a little closer at the probable nature of ths 
differences between species which induce sterility in first crosses 
and in hybrids. In the case of first crosses, the greater or lesa 
difficulty in effecting an union and in obtaining offspring apparently 
depends on several distinct causes. Theie must sometimes be a 


physical impossibility in the male element reaching the ovule, as 
would be the case with a plant having a pistil too long for the polkn- 
tubes to reach the ovarium. It has also been observed that when 
the pollen of one species is placed on the stigma of a distantly allied 
species, though the pollen-tubes protrude, they do not penetrate the 
stigmatic surface. Again, the male element may reach the female 
element but be incapable of causing an embryo to be developed, as 
seems to have been the case with some of Thuret's experiments on 
Fuci. No explanation can be given of these facts, any more than 
why certain trees cannot be grafted on others. Lastly, an embryo 
may be developed, and then perish at an early period. This latter 
alternative has not been sufficiently attended to ; but I believe, 
fi'om observations communicated to me by Mr. Hewitt, who has 
had great experience in hybridising pheasants and fowls, that the 
early death of the embryo is a very frequent cause of sterility in 
first crosses. Mr. Salter has recently given the results of an 
examination of about 500 eggs produced from various crosses 
between three species of Gallus and their hybrids ; the majority of 
these eggs had been fertilised ; and in the majority of the fertilised 
eggs, the embryos had either been partially developed and had then 
perished, or had become nearly mature, but the young chickens had 
been unable to break through the shell. Of the chickens which 
were born, mere than four-fifths died within the first few days, or 
at latest weeks, " without any obvious cause, apparently from mere 
inability to live ;" so that from the 500 eggs only twelve chickens 
were reared. With plants, hybridised embryos probably often 
perish in a like manner ; at least it is known that hybrids raised 
from very distinct species are sometimes weak and dwarfed, and 
perish at an early age ; of which fact Max Wichura has recently 
given some striking cases with hybrid willows. It may be here 
worth noticing that in some cases of parthenogenesis, the embryos 
within the eggs of silk mcths v^hich had not been fertilised, pass 
through their early stages of development and then perish like the 
embryos produced by a cross between distinct species. Until 
becoming acquainted with these facts, I was unwilling to believe in 
the frequent early death of hybrid embryos ; for hybrids, when 
once born, are generally healthy and long-lived, as we see in the 
case of the common mule. Hybrids, however, are differently cir- 
cumstanced before and after birth : when born and living in a 
country where their two parents live, they are generally placed under 
suitable conditions of life. But a hybrid partakes of only half of 
the nature and constitution of its mother ; it may therefore before 
birth, as long as it is nourished within its mother's womb, or within 


tlie egg or seed produced by the motlier, be exposed to conditions 
in some degree unsuitable, and consequently be liable to perish at 
an early period; more especially as all very young beings are 
eininently sensitive to injurious or unnatural conditions of life. 
But after all, the cause more probably lies in some imperfection in 
the original act of impregnation, causing the embryo to be im- 
perfectly developed, rather than in the conditions to which it is 
subsequently exposed. 

In regard to the sterility of hybrids, in vrhich the sexual elements 
are imperfectly developed, the case is somewhat different. I have 
more than once alluded to a large body of facts showing that, when 
animals and plants are removed from their natural conditions, they 
are extremely liable to have their reproductive systems seriously 
affected. This, in fact^ is the great bar to the domestication of 
animals. Between the sterility thus superinduced and that of 
hybrids, there are many points of similarity. In both cases the 
sterility is independent of general health, and is often accompanied 
by excess of size or great luxuriance. In both cases the sterility 
occurs in various degrees ; in both, the male element is the most 
liable to be affected; but sometimes the female more than the 
male. In both, the tendency goes to a certain extent with sys- 
tematic affinity, for whole groups of animals and plants are rendered 
impotent by the same unnatural conditions ; and whole groups 
of species tend to produce sterile hybrids. On the other hand, one 
species in a group will sometimes resist great changes of conditions 
with unimpaired fertility; and certain species in a group will 
produce unusually fertile hybrids. No one can tell, till he tries, 
whether any particular animal will breed under confinement, or 
any exotic plant seed freely under culture ; nor can he tell till he 
tries, whether any two species of a genus will produce more or less 
sterilo hybrids. Lastly, when organic beings are placed during 
several generations under conditions not natural to them, they are 
extremely liable to vary, which seems to be partly due to their 
reproductive systems having been specially affected, though in a 
lesser degtee than when sterility ensues. So it is with hybrids, for 
their offspring in successive generations are eminently liable to vary, 
as every experimentalist has observed. 

Thus we see that when organic beings are placed under new 
and unnatural conditions, and when hybrids are produced by the 
unnatural crossing of two species, the reproductive system, inde- 
pendently of the general state of health, is affected in a very 
similar manner. In the one case, the conditions of life have been 
disturbed, though often in so slight a degree as to be inappreciable 


by us ; in the other case, or that of hybrids, the external conditions 
have remained the same, but the organisation has been disturb>3d 
by two distinct structures and constitutions, including of course the 
reproductive systems, having been blended into one. For it is 
scarcely possible that two organisations should be compounded 
into one, without some disturbance occurring in the development, or 
periodical action, or mutual relations of the different parts and 
organs one to another or to the conditions of life. When hybrids 
are able to breed inter se, they transmit to their offspring from 
generation to generation the same compounded organisation, and 
hence we need not be surprised that their sterility, though in some 
degree variable, does not diminish ; it is even apt to increase, this 
being generally the result, as before explained, of too close inter- 
breeding. The above view of the sterility of hybrids being caused 
by two constitutions being compounded into one has been strongly 
maintained by Max Wichura. 

It must, however, be owned that we cannot understand, on the 
above or any other view, several facts with respect to the sterility 
of hybrids ; for instance, the unequal fertility of hybrids produced 
from reciprocal crosses ; or the increased sterility in those hybrids 
which occasionally and exceptionally resemble closely either pure 
parent. Nor do I pretend that the foregoing remarks go to the 
root of the matter ; no explanation is offered why an organism, 
when placed under unnatural conditions, is rendered sterile. All 
that I have attempted to show is, that in two cases, in some respects 
allied, sterility is the common result,— in the one case from the 
conditions of life having been disturbed, in the other case from 
the organisation having been disturbed by two organisations being 
compounded into one. 

A similar parallelism holds good with an allied yet veiy different 
class of facts. It is an old and almost universal belief founded on a 
considerable body of evidence, which I have elsewhere given, that 
slight changes in the conditions of life are beneficial to all living 
things. We see this acted on by farmers and gardeners in their 
frequent exchanges of seed, tubers, &c., from one soil or climate to 
another, and back again. During the convalescence of animals, 
great benefit is derived from almost any change in their habits of life. 
Again, both with plants and animals, there is the clearest evidence 
that a cross between individuals of the same species, which differ to 
a certain extent, gives vigour and fertility to the offspring ; and 
that close interbreeding continued during several generations between 
the nearest relations, if these be kept under the same conditions of 
life, almost always leads to decreased size, weakness, or sterility. 


Hence it seems that, on the one hand, slight changes in the con- 
ditions of life benefit all organic beings, and on the other hand, that 
slight crosses, that is crosses between the males and females of the 
same species, which have been subjected to slightly diffei'ent con- 
ditions, or which have slightly varied, give vigour and fertility to 
the offspring. But, as we have seen, organic beings long habituated 
to certain uniform conditions under a state of nature, when sub- 
jected, as under confinement, to a considerable change in their 
conditions, very frequently are rendered more or less sterile ; and 
we know that a cross between two forms, that have become widel7 
or specifically different, produce hybrids which are almost always in 
some degree sterile. I am fully persuaded that this double paral- 
lelism is by no means an accident or an illusion. He who is able 
to explain why the elephant and a multitude of other animals are 
incapable of breeding when kept under only partial confinement in 
their native country, will be able to explain the primary cause of 
hybrids being so generally sterile. He will at the same time be 
able to explain hov/ it is that the races of some of our domesticated 
animals, which have often been subjected to new and not uniform 
conditions, are quite fertile together, although they are descended 
from distinct species, which would probably have been sterile if 
aboriginally crossed. The above two parallel series of facts seem 
to be connected together by some common but unknown bond, 
which is essentially related to the principle of life ; this principle, 
according to Mr. Herbert Spencer, being that life depends on, or 
consists in, the incessant action and reaction of various forces, 
which, as throughout nature, are always tending towards an equi- 
librium; and when this tendency is slightly disturbed by any 
change, the vital forces gain in power. 

Reciprocal Dimorphism and Trimorjpldsm. 

This subject may be here briefly discussed, and will be found to 
throw some light on hybridism. Several plants belonging to 
distinct orders present two forms, which exist in about equal 
numbers and which differ in no respect except in their reproductive 
organs ; one form having a long pistil with short stamens, the other 
a short pistil with long stamens ; the two having differently sized 
pollen-grains. With trimorphic plants there are three forms like- 
wise differing in the lengths of their pistils and stamens, in the size 
and colour of the pollen-grains, and in some other respects ; and as 
in each of the three forms there are two sets of stamens, the three 
foi-ms possess altogether six sets of stamens and three kinds of 
pistils. These organs are so proportioned iji length to each other^ 


that half the stamens in two of the forms stand on a level with 
the stigma of the third form. Now I have shown, and the result 
has been confirmed by other observers, that, in order to obtain full 
fertility with these plants, it is necessary that the stigma of the one 
form should be fertilised by pollen taken from the stamens of cor- 
responding height in another form. So that with dimorphic species 
'WO unions, which may be called legitimate, are fully fertile ; and 
two, which may be called illegitimate, are more or less infertile. 
With trimorphic species six unions are legitimate or fully fertile, 
and twelve are illegitimate or more or less infertile. 

ITie infertility which may be observed in various dimorphic and 
trimorphic plants, when they are illegitimately fertilised, that is by 
pollen taken from stamens not corresponding in height with the 
pistil, dififers much in degree, up to absolute and utter sterility ; just 
in the same manner as occurs in crossing distinct species. As the 
degree of sterility in the latter case depends in an eminent degree 
on the conditions of life being more or less favourable, so I have 
found it with illegitimate unions. It is well known that if pollen 
of a distinct species be placed on the stigma of a flower, and its own 
pollen be afterwards, even after a considerable interval of time, 
placed on the same stigma, its action is so strongly prepotent that 
it generally annihilates the effect of the foreign pollen ; so it is with 
the pollen of the several forms of the same species, for legitimate 
pollen is strongly prepotent over illegitimate pollen, when both are 
placed on the same stigma. I ascertained this by fertilising several 
flowers, first illegitimately, and twenty-four hours afterwards legiti- 
mately, with pollen taken from a peculiarly coloured variety, and 
all the seedlings were similarly coloured ; this shows that the 
legitimate pollen, though applied twenty-four hours subsequently, 
had wholly destroyed or prevented the action of the previously 
applied illegitimate pollen. Again, as in making reciprocal crosses 
between the same two species, there is occasionally a great difference 
in the result, so the same thing occurs with trimorphic plants ; for 
instance, the mid-styled form of Lythrum salicaria was illegitimately 
fertilised with the greatest ease by pollen from the longer stamens 
of the short-styled form, and yielded many seeds ; but the latter 
form did not yield a single seed when fertilised by the longer 
stamens of the mid-styled form. 

In all these respects, and in others which might be added, the 
forms of the same undoubted species when illegitimately united 
behave in exactly the same manner as do two distinct species whei: 
crossed. This led me carefully to observe during four years many 
seedlings, raised from several illegitimate unions. The chief result is 


tliat these illegitimate plants, as they may be called, are not fully fer- 
tile. It is possible to raise from dimorphic species, both long-styled 
and short-styled illegitimate x:)laiits, and from trimorphic plants all 
three illegitimate forms. These can then be properly united in a 
legitimate manner. When this is done, there is no apparent reason 
why they should not yield as many seeds as did their parents when 
legitimately fertilised. But such is not the case. They are all 
infertile, in various degrees ; some being so utterly and incurably 
sterile that they did not yield during four seasons a single seed or 
even seed-capsule. The sterility of these illegitimate plants, when 
united with each other in a legitimate manner, may be strictly 
compared with that of hybrids when crossed inter se. If, on the 
other hand, a hybrid is crossed with either pure parent-species, the 
sterility is usually much lessened : and so it is when an illegitimate 
plant is fertilised by a legitimate plant. In the same manner ap 
the steriUty of hybrids does not always run parallel with the 
difEculty of making the first cross between the two parent-species, 
so the sterility of certain illegitimate plants was unusually great, 
whilst the sterility of the union from which they were derived was 
by no means great. With hybrids raised from the same seed- 
capsule the degree of sterility is innately variable, so it is in a 
marked manner with illegitimate plants. Lastly, many hybrids are 
profuse and persistent flowerers, whilst other and more sterile 
hybrids produce few flowers, and are weak, miserable dwarfs; 
exactly similar cases occur with the illegitimate offspring of various 
dimorphic and trimorphic plants. 

Altogether there is the closest identity in character and behaviour 
between illegitimate plants and hybrids. It is hardly an exagge- 
ration to maintain that illegitimate plants are hybrids, produced 
within the limits of the same species by the improper imion ot 
certain forms, whilst ordinary hybrids are produced from an im- 
proper union between so-called distinct species. We have also 
already seen that there is the closest similarity in all respects 
between first illegitimate unions and first crosses between distinct 
species. "l.'his will perhaps be made more fully apparent by an 
illustration ; we may suppose that a botanist found two well- 
marked varieties (and such occur) of the long-styled form of the 
trimorphic Lythrum salicaria, and that he determined to try by 
crossing whether they were specifically distinct. He would find 
that they yielded only about one-fifth of the proper number of seed, 
and that they behaved in all the other above specified respects as if 
they had been two distinct species. But to make the case sure, he 
would raise plants from his supposed hybridised seed, and he would 


find that the seedlings were miserably dwarfed and utterly sterik, and 
that they behaved in all other respects like ordinary hybrids. He 
might then maintain that he had actually proved, in accordance with 
the common view, that his two varieties were as good and as distinct 
species as any in the world ; but he would be completely mistaken. 

The facts now given on dimorphic and trimorphic plants are 
important, because they show us, first, that the physiological test 
of lessened fertility, both in first crosses and in hybrids, is no safe 
criterion of specific distinction ; secondly, because we may conclude 
that there is some unknown bond which connects the infertility of 
illegitimate unions with that of their illegitimate offspring, and we 
are led to extend the same view to first crosses and hybrids ; 
thirdly, because we find, and this seems to me of especial importance, 
that two or three forms of the same species may exist and may 
differ in no respect whatever, either in structure or in constitution, 
relatively to external conditions, and yet be sterile when united in 
certain ways. For we must remember that it is the union of the 
sexual elements of individuals of the same form, for instance, of two 
long-styled forms, which results in sterility ; whilst it is the union 
of the sexual elements proper to two distinct forms which is fertile. 
Hence the case appears at first sight exactly the reverse of what 
occurs, in the ordinary unions of the individuals of the same species 
and with crosses between distinct species. It is, however, doubtful 
whether this is really so ; but I will not enlarge on this obscure 

We may, however, infer as probable from the consideration of 
dimorphic and trimorphic plants, that the sterility of distinct 
species when crossed and of their hybrid progeny, depends exclu- 
sively on the nature of their sexual elements, and not on any differ- 
ence in their structure or general constitution. We are also led 
to this same conclusion by considering reciprocal crosses, in which 
the male of one species cannot be united, or can be united with 
great difficulty, with the female of a second species, whilst the 
converse cross can be effected with perfect facility. That excellent 
observer, Gartner, likewise concluded that species when crossed are 
sterile owing to differences confined to their reproductive systems. 

Fertility of Varieties when Crossed, and of their Mongrel 
Offspring, not universal. 

It may be urged, as an overwhelming argument, that there must 
be some essential distinction between species and varieties, inasmuch 
as the latter, however much they may differ from each other in 
QTitrrnal appearance* cross with perfect facility, and yield perfectly 


fertile offspring. With some exceptions, presently to be given, I 
fully admit that this is the rule. But the subject is surrounded by 
diflBculties, for, looking to varieties produced under nature, if two 
forms hitherto reputed to be varieties be found in any degree sterile 
together, they are at once ranked by most naturalists as species. 
For instance, the blue and red pimpernel, which are considered by 
most botanists as varieties, are said by Gartner to be quite sterile 
when crossed, and he consequently ranks them as undoubted 
species. If we thus argue in a circle, the fertility of all varieties 
produced under nature will assuredly have to be granted. 

If we turn to varieties, produced, or supposed to have been pro- 
duced, under domestication, we are still involved in some doubt. 
For when it is stated, for instance, that certain South American 
indigenous domestic dogs do not readily unite with European dogs, 
the explanation which will occur to every one, and probably the 
true one, is that they are descended from aboriginally distinct 
species. Nevertheless the perfect fertility of so many domestic 
races, differing widely from each other in appearance, for instance 
those of the pigeon, or of the cabbage, is a remarkable fact ; more 
especially when we reflect how many species there are, which, 
though resembling each other most closely, are utterly sterile when 
intercrossed. Several considerations, however, render the fertility 
of domestic varieties less remarkable. In the .first place, it may 
be observed that the amount of external difference between two 
species is no sure guide to their degree of mutual sterility, so that 
similar differences in the case of varieties would be no sure guide. 
It is certain that with species the cause lies exclusively in differ- 
ences in their sexual constitution. Now the varying conditions 
to which domesticated animals and cultivated plants have been 
subjected, have had so little tendency towards modifying the 
reproductive system in a manner leading to mutual sterility, that 
we have good grounds for admitting the directly opposite doctrine 
of Pallas, namely, that such conditions generally eliminate this 
tendency ; so that the domesticated descendants of species, which in 
their natural state probably would have been in some degree sterile 
when crossed, become perfectly fertile together. With plants, so 
far is cultivation from giving a tendency towards sterility between 
distinct species, that in several well-authenticated cases already 
alluded to, certain plants have been affected in an opposite manner, 
for they have become self-impotent, whilst still retaining the 
capacity of fertilising, and being fertilised by, other species. If 
the Pallasian doctrine of the elimination of sterility through long- 
3ontinued domestication be admitted, and it can hardly be rejecteii, 


it becomes in the liighest degree improbable that similar conditions 
long-continued should likewise induce this tendency; though in 
certain cases, with species having a peculiar constitutioa, sterility 
might occasionally be thus caused. Thus, as I believe, we can 
understand why with domesticated animals varieties have not been 
produced which are mutually sterile ; and why with plants only a 
few such cases, immediately to be given, have been observed. 

The real difficulty in our present subject is not, as it appears to 
me, why domestic varieties have not become mutually infertile 
when crossed, but why this has so generally occurred with naturla 
varieties, as soon as they have been permanently modified in a 
sufficient degree to take rank as species. We are far from precisely 
knowing the cause ; nor is this surprising, seeing how profoundly 
ignorant we are in regard to the normal and abnormal action of 
the reproductive system. But we can see that species, owing to 
their struggle for existence with numerous competitors, will have 
been exposed during long periods of time to more uniform con- 
ditions, than have domestic varieties ; and this may well make a 
wide difference in the result. For we know how commonly wild 
animals and plants, when taken from their natui-al conditions and 
subjected to captivity, are rendered sterile ; and the reproductive 
functions of organic beings which have always lived under natural 
conditions would probably in like manner be eminently sensitive 
to the influence of an unnatural cross. Domesticated productions, 
on the other hand, which, as shown by the mere fact of their 
domestication, were not originally highly sensitive to changes in 
their conditions of life, and which can now generally resist with 
undiminished fertility repeated changes of conditions, might be 
expected to produce varieties, which would be little liable to have 
their reproductive powers injuriously affected by the act of crossing 
with other varieties which had originated in a like manner. 

I have as yet spoken as if the varieties of the same species were 
invariably fertile when intercrossed. But it is impossible to resist 
the evidence of the existence of a certain amount of sterility in the 
few following cases, which 1 will briefly abstract. The evidence is 
at least as good as that from which we believe in the sterility of a 
multitude of species. The evidence is, also, derived from hostile 
witnesses, who in all other cases consider fertility and sterility as 
s&fe criterions of specific distinction. Gartner kept during several 
years a dwarf kind of maize with yellow seeds, and a tall variety 
v/ith red seeds growing near each other in his garden ; and although 
these plants have separated sexes, they never naturally crossed. 
He then fertilised thirteen flowers of the one kind with poUec of the 


other; but only a single head produced any seed, and this one 
head produced only five grains. Manipulation in this case could 
not have been injurious, as the plants have separated sexes. No 
one, I believe, has suspected that these varieties of maize are 
distinct species; and it is important to notice that the hybrid 
plants thus raised were themselves 'perfectly fertile ; so that even 
Gartner did not venture to consider the two varieties as specifically 

Girou de Buzareingues crossed three varieties of gourd, which 
like the maize has separated sexes, and he asserts that their mutual 
fertilisation is hy so much the less easy as their differences are 
greater. How far these experiments may be trusted, I know not ; 
but the forms experimented on are ranked by Sageret, who mainly 
founds his classification by the test of infertility, as varieties, and 
jSTaudin has come to the same conclusion. 

The following case is far more remarkable, and seems at first 
incredible ; but it is the result of an astonishing number of experi- 
ments made during many years on nine species of Verbascum, by 
so good an observer and so hostile a witness as Gartner : namely 
that the yellow and white varieties when crossed produce less seed 
than the similarly coloured varieties of the same species. Moreover, 
he asserts that, when yellow and white varieties of one species are 
crossed with yellow and white varieties of a distinct species, 
more seed is produced by the crosses between the similarly 
coloured flowers, than between those which are differently coloured. 
Mr. Scott also has experimented on the species and varieties ol 
Verbascum ; and although unable to confirm Gartner's results on 
the crossing of the distinct species, he finds that the dissimilarly 
coloured varieties of the same species yield fewer seeds, in the pro- 
|X)rtion of 86 to 100, than the similarly coloured varieties. Yet these 
varieties differ in no respect except in the colour of their flowers ; 
and one variety can sometimes be raised from the seed of another. 

Kolreuter, whose accuracy has been confirmed by every subsequent 
observer, has proved the remarkable fact, that one particular 
variety of the common tobacco was mors fertile than the other 
varieties, when crossed with a widely distinct species. He experi- 
mented on five forms which are commonly reputed to be varieties, 
and which he tested by the severest trial, namely, by reciprocal 
crosses, and he found their mongrel offspring perfectly fertile. But 
one of these five varieties, when used either as the father or mother 
and crossed with the Nicotiana glutinosa, always yielded hybrids not 
so sterile as those which were produced from the four other varieties 
wbea GToesed with N. gl^ibinosa. Hence the reproductive system 


of this one variety must have "been in some manner and in some 
degree modified. 

]*'rom these facts it can no longer be maintained that varieties 
when crossed are invariably quite fertile. From the great difficulty 
of ascertaining the infertility of varieties in a state of nature, for 
a supposed variety, if proved to be infertile in any degree, would 
almost universally bo ranked as a species ; — from man attending 
only to external characters in his domestic varieties, and from such 
varieties not having been exposed for very long periods to uniform 
conditions of Ufe ; — from these several considerations we may con- 
clude that fertility does not constitute a fundamental distinction 
between varieties and species when crossed. The general sterility 
of crossed species may safely be looked at, not as a special acquire- 
ment or endowment, but as incidental on changes of an unknown 
nature in their sexual elements. 

Hybrids and Mongrels compared, independently of their 

Independently of the question of fertility, the offspring of species 
and of varieties when crossed may be compared in several other 
respects. Gartner, whose strong wish it was to draw a distinct line 
between species and varieties, could find very few, and, as it seems 
to me, quite unimportant differences between the so-called hybrid 
ofifsjjring of species, and the so-called mongrel offspring of varieties. 
And, on the other hand, they agree most closely in many important 

I shall here discuss this subject with extreme brevity. The most 
important distinction is, that in the first generation mongrels are 
more variable than hybrids ; but Gartner admits that hybrids from 
species which have long been cultivated are often variable in the 
first generation ; and I have myself seen striking instances of this 
fact. Gartner further admits that hybrids between very closely 
Jellied species are more variable than those from very distinct 
species ; and this shows that the difference in the degree of varia- 
bility graduates away. When mongrels and the more fertile hybrids 
are propagated for several generations, an extreme amount of varia- 
bility in the offspring in both cases is notorious ; but some few 
instances of both hybrids and mongrels long retaining a uniform 
character could be given. The variability, however, in the succes- 
sive generations of mongrels is, perhaps, greater than in hybrids. 

This greater variability in mongrels than in hybrids does not 
Beem at all surprising. For the parents of mongrels are varieties, 
and mostly domestic varistietj (very few experiments having been 

B 2 


tried on natural varieties), and this implies that there has beon 
recent variability, which would often continue and would augment 
that arising from the act of crossing. The slight variability of 
hybrids in the first generation, in contrast with that in the succeed- 
ing generations, is a curious fact and deserves attention. For ii 
bears on the view which I have taken of one of the causes of 
ordinary variability; namely, that the reproductive system from 
being eminently sensitive to changed conditions of life, fails under 
these circumstances to perform its proper function of producing 
offspring closely similar in all respects to the parent-form. Now 
hybrids in the first generation are descended from species (excluding 
those long-cultivated) which have not had their reproductive 
systems in any way affected, and they are not variable ; but 
hybrids themselves have their reproductive systems seriously 
affected, and their descendants are highly variable. 

But to return to our comparison of mongrels and hybrids: 
Glirtner states that mongrels are more liable than hybrids to revert 
to either parent-form ; but this, if it be true, is certainly only a dif- 
ference in degree. Moreover, Gartner expressly states that hybrids 
from long cultivated plants are more subject to reversion thair 
hybrids from species in their natural state ; and this probably 
explains the singular difference in the results arrived at by different 
observers : thus, Max Wicbura doubts whether hybrids ever revert 
to their parent-forms, and ±ie experimented on uncultivated species 
of willows; whilst Naudin, on the other hand, insists in the strongest 
terms on the almost universal tendency to reversion in hybrids, 
and he experimented chiefly on cultivated plants. Gartner further 
states that when any two species, although most closely allied to 
each other, are crossed with a third species, the hybrids are widely 
different from each other ; whereas, if two very distinct varieties of 
one species are crossed with another species, the hybrids do not 
differ much. But this conclusion, as far as I can make out, is 
founded on a single experiment ; and seems directly opposed to the 
results of several experiments made by Kolreuter. 

Buch alone are the unimportant differences which Gartner is able 
to point out between hybrid and mongrel plants. On the other 
hand, the degrees and kinds of resemblance in mongrels and in 
hybrids to their respective parents, more especially in hybrids pro- 
duced from nearly related species, follow according to Gartner the 
same laws. When two species are crossed, one has sometimes a 
prepotent power of impressing its likeness on the hybrid. So I 
believe it to be with varieties of plants; and with animals one 
variety certainly often has this prepotent power over another 


irariety. Hybrid plants pioduced from a reciprocal cross, generally 
resemble each other closely ; and so it is with mongrel plants from 
a reciprocal cross. Both hybrids and mongrels can be reduced tc 
either pure parent-form, by repeated crosses in successive generations 
with either parent. 

These several remarks are apparently applicable to animals ; but 
the subject is here much complicated, partly owing to the existence 
of secondary sexual characters ; but more especially owing to pre- 
potency in transmitting likeness running more strongly in one sex 
than in the other, both when one species is crossed with another, 
and when one variety is crossed with another variety. For instance, 
[ think those authors are right, who maintain that the ass has a 
prepotent power over the horse, so that both the mule and the hinny 
resemble more closely the ass than the horse ; but that the pre- 
potency runs more strongly in the male than in the female ass, so 
that the mule, which is the oifspring of the male ass and mare, is 
more like an ass, than is the hinny, which is the offspring of the 
female-ass and stallion. 

Much stress has been laid by some authors on the supposed fact, 
that it is only with mongrels that the offspring are not intermediate 
in character, but closely resemble one of their parents ; but this does 
sometimes occur with hybrids, yet I grant much less frequently 
than with mongrels. Looking to the cases which I have collected 
of cross-bred animals closely resembling one parent, the resemblances 
seem chiefly confined to characters almost monstrous in their nature, 
and which have suddenly appeared — such as albinism, melanism, 
■deficiency of tail or horns, or additional fingers and toes ; and do not 
relate to characters which have been slowly acquired through selec- 
tion. A tendency to sudden reversions to the perfect character of 
cither parent would, also, be much more likely to occur with mongrels, 
which are descended from varieties often suddenly produced and 
semi-monstrous in character, than with hybrids, which are descended 
from species slowly and naturally produced. On the whole, I 
-entirely agree with Dr. Prosper Lucas, who, after arranging an 
enormous body of facts with respect to animals, comes to the con- 
clusion, that the laws of resemblance of the child to its parents are 
the same, whether the two parents differ little or much from each 
uther, namely, in the union of individuals of the same variety, or of 
different varieties, or of distinct species. 

Independently of the question of fertility and sterility, in all 
other respects there seems to be a general and close similarity in 
the offspring of crossed species, and of crossed varieties. If we 
look at species as having been specially created, and at varieties as 

262 SUMMARY. [Chap. JX. 

having been produced by secondary laws, this similarity would be 
an astonishing fact. But it harmonises perfectly with the view 
that there is no essential distinction between species and varieties. 

Summary of Chapter. 

First crosses between forms, sufiSciently distinct to be ranked as 
species, and their hybrids, are very generally, but not universally 
sterile. The sterility is of all degrees, and is often so slight that the 
most careful experimentalists have arrived at diametrically opposite 
conclusions in ranking forms by this test. The sterility is innately 
variable in individuals of the same species, and is eminently 
susceptible to the action of favourable and unfavourable conditions. 
The degree of sterility does not strictly follow systematic affinity, 
but is governed by several curious and complex laws. It is generally 
different, and sometimes widely different, in reciprocal crosses between 
the same two species. It is not always equal in degree in a first 
cross and in the hybrids produced from this cross. 

In the same manner as in grafting trees, the capacity of one 
species or variety to take on another, is incidental on Ufferences, 
generally of an imknown nature, in their vegetative sysiems, so in 
crossing, the greater or less facility of one species to unite with 
another is incidental on unknown differences in their reproductive 
systems. There is no more reason to think that species have been 
specially endowed with various degrees of sterility to prevent their 
crossing and blending in nature, than to think that trees have been 
specially endowed with various and somewhat analogous degrees 
of difficulty in being grafted together in order to prevent their 
inarching in our forests. 

The sterility of first crosses and of their hybrid progeny has not 
been acquired through natural selection. In the case of first crosses 
it seems to depend on several circumstances ; in some instances in 
chief part on the early death of the embryo. In the case of hybrids, 
it apparently depends on their whole organisation having been 
disturbed by being compounded from two distinct forms ; the 
sterility being closely allied to that which so frequently affects pure 
species, when exposed to new and unnatural conditions of life, 
tie who will explain these latter cases will be able to explain the 
sterility of hybrids. This view is strongly supported by a parallelism 
of another kind : namely, that, firstly, slight changes in the con- 
ditions of life add to the vigour and fertility of all organic beings ; 
and secondly, that the crossing of forms, which have been exposed -do 
slightly difierent conditions of life or which have varied, favours the 
size, vigour, and fertility of their offspring. The facts given on the 

Chap. IX.] SUMMARY. 263 

bterility OJ" the illegitimate unions of dimorphic and trimorphio 
plants and of their illegitimate progeny, perhaps render it probable 
that some unknown bond in all cases connects the degree of fertility 
of first unions with that of their offspring. The consideration of these 
facts on dimorphism, as well as of the results of reciprocal crosses, 
clearly leads to the conclusion that the primary cause of the sterility 
of crossed spo-cies is confined to differences in their sexual elements. 
But why, in xhe case of distinct species, the sexual elements should 
80 generally have become more or less modified, leading to their 
mutual infertility, we do not know ; but it seems to stand in some 
close relation to species having been exposed for long periods oi 
time to nearly uniform conditions of life. 

It is not surprising that the difficulty in crossing any two species, 
and the sterility of their hybrid off'spring, should in most cases cor- 
respond, even if due to distinct causes; for both depend on the 
amount of difference between the species which are crossed. Nor 
is it surprising that the facility of effecting a first cross, and the 
fertihty of the hybrids thus produced, and the capacity of being 
grafted together — though this latter capacity evidently depends on 
widely different circumstances — should all run, to a certain extent, 
parallel with the systematic affinity of the forms subjected to expe- 
riment ; for systematic affinity includes resemblances of all kinds. 

First crosses between forms known to be varieties, or sufficiently 
alike to be considered as varieties, and their mongrel offspring, are 
very generally, but not, as is so often stated, invariably fertile. 
Nor is this almost universal and perfect fertility surprising, when 
it is remembered how liable we are to argue in a circle with respect 
to varieties in a state of nature ; and when we remember that the 
greater number of varieties have been produced under domestication 
by the selection of mere external differences, and that they have 
not been long exposed to uniform conditions of life. It should also 
be especially kep: in mind, that long-continued domestication tends 
to eliminate sterility, and is therefore little likely to induce this 
same quality. Independently of the question of fertility, in all 
other respects there is the closest general resemblance between 
hybrids and mongrels, — in their variability, in their power of 
absorbing each other by repeated crosses, and in their inheritance 
of characters from both parent-forms. Finally, then, although we 
are as ignorant of the precise cause of the sterility of first crosses 
and of hybrids as we are why animals and plants removed from 
their natural conditions become sterile, yet the facts given in this 
chapter do not seem to ine opposed to the belief that species 
aborigirially existed as varieties. 

State niS!'»nrHS dV^O 



On the Imperfection of the Geological Eecord. 

On the absence of intermediata varieties at the present day — On the nature 
of extinct intermediate varieties; on their number — On the lapse ol 
time, as 'inferred from the I'ate of denudation and of deposition — On 
the lapse of time as estimated by years — On the poorness of our palaeon- 
tological collections — On the intermittence of geological formations — 
On the denudation of granitic areas — On the absence of intermediate 
varieties in any one formation — On the sudden appearance of groups of 
species — On their sudden a^ pearance in the lowest known fossiliferoiu 
strata — Antiquity of the habitable earth. 

In the sixth chapter I enumerated the chief objections which might 
be justly urged against the views maintained in this volume. 
Most of them have now been discussed. One, namely the dis- 
tinctness of specific forms, and their not being blended together 
by innumerable transitional links, is a very obvious difficulty. 
I assigned reasons why such links do not commonly occur at the 
present day under the circumstances apparently most favourable for 
their presence, namely on an extensive and continuous area with 
graduated physical conditions. I endeavoured to show, that the 
life of each species depends in a more important manner on tho 
presence of other already defined organic forms, than on climate 
and, therefore, that the really governing conditions of life do not 
graduate away quite insensibly like heat or moisture. I endea- 
voured, also, to show that intermediate varieties, from existing in 
lesser numbers than the forms which they connect, will generally 
l>e beaten out and exterminated during the course of further modifi- 
cation and improvement. The main cause, however, of innumerable 
intermediate links not now occurring everywhere throughout nature, 
depends on the very process of natural selection, through which new 
varieties continually take the places of and supplant their parent- 
t jrms. But just in proportion as this process of extermination has 
BCted on an enormous scale, so must the number of intermediate 
varieties, which have formerly existed, be truly enormous. Why 
then is not every geological formation and every stratum full cf 


such intermediate links? Geology assuredly does not reveal any 
Buch finely-graduated organic chain ; and this, perhaps, is the most 
obvious and serious objection which can be urged against the theory. 
The explanation lies, as I believe, in the extreme imperfection ol 
the geological record. 

In the first place, it should always be borne in mind what sort 
of intermediate forms must, on the theory, have formerly existed. 
I have found it difficult, when looking at any two species, to avoid 
picturing to myself forms directly intermediate between them. But 
this is a wholly false view ; we should always look for forms inter- 
mediate between each species and a common but unknown pro- 
genitor ; and the progenitor will generally have differed in some 
respects from all its modified descendants. To give a simple 
illustration : the fantail and pouter pigeons are both descended 
from the rock-pigeon ; if we possessed all the intermediate varieties 
which have ever existed, we should have an extremely close series 
between both and the rock-pigeon ; but we should have no varieties 
directly intermediate betw^een the fantail and pouter ; none, for 
instance, combining a tail somewhat expanded with a crop some- 
what enlarged, the characteristic features of these two breeds. 
These two breeds, moreover, have become so much modified, 
that, if we had no historical or indirect evidence regarding their 
origin, it would not have been possible to liave determined, from a 
mere comparison of their structure with that of the rock-pigeon, 
C. livia, whether they had descended from this species or from some 
other allied form, such as C. oenas. 

So with natural species, if we look to forms very distinct, for 
instance to the horse and tapir, we have no reason to suppose that 
links directly interniediate between them ever existed, but between 
each and an unknown common parent. The common parent will 
have had in its whole organisation much general resemblance to the 
tapir and to the horse ; but in some points of structure may have 
differed considerably from both, even perhaps more than they 
differ from each other. Hence, in all such cases, we should bo 
unable to recognise the parent-form of any tw^o or more species, 
even if we closely compared the structure of the parent with that of 
its modified descendants, unless at the same time we had a nearly 
perfect chain of the intermediate links. 

It is just possible by the theory, that one of two living forma 
might have descended from the other; for instance, a horse from a 
tapir ; and in this case direct intermediate links will have existed 
between them. But such a case would imply that one form had 
remained for a very long period unaltered, whilst its descendantft 

266 THE LAPSE OF TIME. [Chap. X. 

had undergone a vast amount of change ; and the principle of com- 
petition between organism and organism, between child and parent, 
will render this a very rare event ; for in all cases the new and 
improved forms of life tend to supplant the old and unimproved 

By the theory of natural selection all living species have been 
connected with the parent-species of each genus, by differences not 
greater than we see between the natural and domestic varieties 
of the same species at the present day ; and these parent- 
species, now generally extinct, have in their turn been similarly 
connected with more ancient forms ; and so on backwards, always 
converging to the common ancestor of each great class. So that 
the number of intermediate and transitional links, between all 
living and extinct species, must have been inconceivably great. But 
assuredly, if this theory be true, such have lived upon the earth. 

On the Lajpse of Time, as inferred from the rate of Deposition and 
extent of Denudation. 

Independently of our not finding fossil remains of such infinitely 
numerous connecting links, it may be objected that time cannot 
have sufficed for so great an amount of organic change, all changes 
having been effected slowly. It is hardly possible for me to 
recall to the reader who is not a practical geologist, the facts 
leading the mind feebly to comprehend the lapse of time. He 
who can read Sir Charles Lyell's grand work on the Principles 
of Geology, which the future historian will recognise as having pro- 
duced a revolution in natural science, and yet does not admit how 
vast have been the past periods of time, may at once close this 
volume. Not that it suffices to study the Principles of Geology, 
or to read special treatises by different observers on separate 
formations, and to mark how each author attempts to give an 
inadequate idea of the duration of each formation, or even of each 
stratum. We can best gain some idea of past time by knowing 
the agencies at work, and learning how deeply the surface of the 
land lias been denuded, and how much sediment has been deposited. 
As Lyell has well remarked, the extent and thickness of our sedi- 
mentary formations are the result and the measure of the denu- 
dation which the earth's crust has elsewhere undergone. Therefore 
a man should examine for himself the great piles of superimposed 
strata, and watch the rivulets bringing down mud, and the waves 
wearing away the sea-cliffs, in order to comprehend something; 
about the duration of past time, the monuments of which wc see 
oil around us. 

Chap. X.] THE LAPSE OF TIME. 267 

It is good to wander along the coast, when formed of moderately 
hard rocks, and mark the process of degradation. The tides in 
most cases reach the cliffs only for a short time twice a day, and 
the waves eat into them only when they are charged with sand or 
pebbles ; for there is good evidence that pure water effects nothing 
in wearing away rock. At last the base of the cliff is undermined, 
huge fragments fall down, and these, remaining fixed, have to be 
worn away atom by atom, until after being reduced in size they 
can be rolled about by the waves, and then they are more quickly 
2;round into pebbles, sand, or mud. But how often do we see along 
the bases of retreating cliffs rounded boulders, all thickly clothed 
by marine productions, showing how little they are abraded and 
how seldom they are rolled about ! Moreover, if we follow for a 
few miles any line of rocky cliff, which is undergoing degradation, 
we find that it is only here and there, along a short length or 
round a promontory, that the cliffs are at the present time suffering. 
The appearance of the surface and the vegetation show that else- 
where years have elapsed since the waters washed their base. 

We have, however, recently learnt from the observations ot 
Ramsay, in the van of many excellent observers — of Jukes, Geikie, 
Croll, and others, that subaerial degradation is a much more im- 
portant agency than coast-action, or the power of the waves. The 
whole surface of the land is exposed to the chemical action of 
the air and of the rain-water with its dissolved carbonic acid, and 
in colder countries to frost ; the disintegrated matter is carried down 
even gentle slopes during heavy rain, and to a greater extent than 
might be supposed, especially in arid districts, by the wind; it 
is then transported by the streams and rivers, which when rapid 
deepen their channels, and triturate the fragments. On a rainy 
day, even in a gently undulating country, we see the effects of 
subaerial degradation in the muddy rills which flow down every 
slope. Messrs. Eamsay and Whitaker have shown, and the ob- 
servation is a most striking one, that the great lines of escarpment 
in the Wealden district and those ranging across England, which 
formerly were looked at as ancient sea-coasts, cannot have been 
thus formed, for each line is composed of one and the same forma- 
tion, whilst our sea-cliffs are everywhere formed by the intersection 
of various formations. This being the case, we are compelled to 
admit that the escarpments ewe their origin in chief part to the 
rocks of which they are composed having resisted subaerial denu- 
dation better than the surrounding surface ; this surface conse- 
quently has been gradually lowered, with the lines of harder rock 


left projecting. Nothing impresses the mind with the vast duration 
of time, according to our ideas of time, more forcibly than the con- 
viction thus gained that subaerial agencies which apparently have 
so little power, and which seem to work so slowly, have produced 
great results. 

When thus impressed with the slow rate at which the land is 
worn away through subaerial and littoral action, it is good, in order 
to appreciate the past duration of time, to consider, on the one hand, 
the masses of rock which have been removed over many exten- 
sive areas, and on the other hand the thickness of our sedimentary 
formations. I remember having been much struck when viewing 
volcanic islands, which have been worn by the waves and pared 
all round into perpendicular cliffs of one or two thousand feet in 
height ; for the gentle slope of the lava-streams, due to their for- 
merly liquid state, showed at a glance how far the hard, rocky 
beds had once extended into the open ocean. The same story is 
told still more plainly by faults, — those great cracks along which 
the strata have been upheaved on one side, or thrown down on 
the other, to the height or depth of thousands of feet ; for since the 
crust cracked, and it makes no great difference whether the up- 
heaval was sudden, or, as most geologists now believe, was slow 
and effected by many starts, the surface of the land has been so 
completely planed down that no trace of these vast dislocations 
is externally visible. The Craven fault, for instance, extends for 
upwards of 30 miles, and along this line the vertical displacement 
of the strata varies from 600 to 3000 feet. Professor Eamsay has 
published an account of a downthrow in Anglesea of 2300 feet ; 
and he informs me that he fully believes that there is one in Merio- 
nethshire of 12,000 feet; yet in these cases there is nothing on 
the surface of the land to show such prodigious movements; the 
pile of rocks on either side of the crack having been smoothly swept 

On the other hand, in all parts of the world the piles of sedi- 
mentary strata are of wonderful thickness. In the Cordillera I esti- 
mated one mass of conglomerate at ten thousand feet ; and although 
conglomerates have probably been accumulated at a quicker rate 
than finer sediments, yet from being formed of worn and rounded 
pebbles, each of which bears the stamp of time, they are good to 
ehow how slowly the mass must have been heaped together. Pro- 
fessor Ramsay has given me the maximum thickness, from actual 
measurement in most cases, of the successive formations in dijfereril 
parts of Great Britain ; and this is the result : — 

Cha?. a.] the lapse of TIME. 269 

PalflBozoic strata (not including Igneous beds) .. .. 57,154 

Secondary strata 13,190 

Tertiary strata 2,240 

— making altogcthsr 72,584 feet ; that is, very neaiiy ttirteen an^ 
three-quarters British miles. Some of the formations, which arc 
represented in England by thin beds, are thousands of feet in 
thickness on the Continent. Moreover, between each successive 
formation, we have, in the opinion of most geologists, blank periods 
of enormous length. So that tlie lofty pile of sedimentary rocks in 
Britain gives but an inadequate idea of the time which has elapsed 
during their accumulation. The consideration of these various facts 
impresses the mind almost in the same manner as does the vain 
endeavour to grapple with the idea of eternity. 

Nevertheless this impression is partly false. Mr. Croll, in an 
interesting paper, remarks that we do not err *' in forming too great 
a conception of the length of geological periods," but in estimating 
them by years. When geologists look at large and complicated 
phenomena, and then at the figures representing several million 
years, the two produce a totally different effect on the mind, and 
the figures are at once pronounced too small. In regard to subaeria) 
denudation, Mr. Croll shows, by calculating the known amount ol 
sediment annually brought down by certain rivers, relatively to their 
areas of drainage, that 1000 feet of solid rock, as it became gradu- 
ally disintegrated, would thus be removed from the mean level of 
the whole area in the course of six million years. This seems an 
astonishing result, and some considerations lead to the suspicion 
that it may be too large, but even if halved or quartered it is still 
very surprising. Few of us, however, know what a million really 
means : Mr. Croll gives the foUow^ing illustration : take a narrow 
strip of paper, 83 feet 4 inches in length, and stretch it along the 
wall of a large hall ; then mark off at one end the tenth of an inch. 
This tenth of an inch will represent one hundred years, and the 
entire strip a million years. But let it be borne in mind, in relation 
to the subject of this work, what a hundred years implies, repre- 
Bented as it is by a measure utterly insignificant in a hall of the 
above dimensions. Several eminent breeders, during a single life- 
time, have so largely modified some of the higher animals, which 
propagate their kind much more slowly than most of the lower 
animals, that they have formed what well deserves to be called a 
new sub-breed. Few men have attended with due care to any one 
strain for more than half a century, so that a hundred years repre- 
sents the work of two breeders in suocessio" It is not to be pui>- 


posed that species in a state of nature ever change so quickly as 
domestic animals under the guidance of methodical selection. The 
comparison would be in every way fairer with the effects which 
follow from unconscious selection, that is the preservation of the 
most useful or beautiful animals, with no intention of modifying 
the breed ; but by this process of unconscious selection, various 
breeds have been sensibly changed in the course of two or three 

Species, however, probably change much more slowly, and within 
the same country only a few change at the same time. This slow- 
ness follows from all the inhabitants of the same country being 
already so well adapted to each other, that new places in the polity 
of nature do not occur until after long intervals, due to the occur- 
rence of physical changes of some kind, or through the immigration 
of new forms. Moreover variations or individual differences of the 
right nature, by which some of the inhabitants might be better 
fitted to their new places under the altered circumstances, would 
not always occur at once. Unfortunately we have no means of 
determining, according to the standard of years, how long a period 
it takes to modify a species ; but to the subject of time we must 

On the Poorness of our Fnlceontological Collections. 

Now let us turn to our richest geological museums, and what 
a paltry display we behold ! That our collections are imperfect is 
admitted by every one. The remark of that admirable palseonto- 
logist, Edward Forbes, should never be forgotten, namely, that very 
many fossil species are known and named from single and often 
broken specimens, or from a few specimens collected on some one 
spot. Only a small portion of the surface of the earth has been 
geologically explored, and no part with sufiScient care, as the im- 
portant discoveries made every year in Europe prove. No organism 
wholly soft can be preserved. Shells and bones decay and disappear 
when left on the bottom of the sea, where sediment is not accumu- 
lating. We probably take a quite erroneous view, when we assume 
that sediment is being deposited over nearly the whole bed of the 
sea, at a rate sufficiently quick to embed and preserve fossil remains. 
Throughout an enormously large proportion of the ocean, the bright 
blue tint of the water bespeaks its purity. The many cases on 
record of a formation conformably covered, after an immense 
interval of time, by another and later formation, without the under- 
lying bed having suffered in the interval any wear and tear, seem 
explicable only on the view of the bottom of the sea not rarely lyin^ 


for ages in an unaltered condition. The remains whicli do becx)me 
embedded, if in sand or gravel, will, when the beds are upraised, 
generally be dissolved by the percolation of rain-water charged with 
carbonic acid. Some of the many kinds of animals which live on 
the beach between high and low water mark seem to be rarely pre- 
BeiTed. For instance, the several species of the Chthamalinse (a 
sub-family of sessile cirripedes) coat the rocks all over the world in 
infinite numbers : they ara all strictly littoral, with the exception 
of a siogle Mediterranean species, which inhabits deep water, and 
this has been found fossil in Sicily, whereas not one other species 
has hitherto been found in any tertiary formation : yet it is known 
that the genus Chthamalus existed during the Chalk period. Lastly, 
many great deposits requiring a vast length of time for their accu- 
mulation, are entirely destitute of organic remains, without our being 
able to assign any reason : one of the most striking instances is 
that of the Flysch formation, which consists of shale and sandstone, 
several thousand, occasionally even six thousand feet, in thickness, 
and extending for at least 300 miles from Vienna to Switzerland ; 
and although this great mass has been most carefully Sf^arched, no 
fossils, except a few vegetable remains, have been found. 

With respect to the terrestrial productions which lived during 
the Secondary and Palaeozoic periods, it is superfluous to state that 
our evidence is fragmentary in an extreme degree. For instance, 
until recently not a land shell was known belonging to either of 
these vast periods, with the exception of one species discovered by 
Sir C. Lyell and Dr. Dawson in the carboniferous strata of North 
America; but now land-shells have been found in the lias. In 
regard to mammiferous remains, a glance at the historical table 
published in Lyell's Manual will bring home the tmth, how acci- 
dental and rare is their preservation, far better than pages of detail. 
Nor is their rarity surprising, when we remember how large a pro- 
portion of the bones of tertiary mammals have been discovered 
either in caves or in lacustrine deposits ; and that not a cave or true 
lacustrine bed is known belonging to the age of our secondary or 
palaeozoic formations. 

But the imperfection in the geological record largely results 
from another and more important cause than any of the foregoing ; 
namely, from the several formations being separated from each 
other by wide intervals of time. This doctrine has been empha- 
tically admitted by many geologists and palasontologists, who, like 
E. Forbes, entirely disbelieve in the change of species. When we 
see the formations tabulated in written works, or when we follow 
them in nature, it is difficult to avoid believing that they are closel j 

272 THE POORNESS OF OUR [Chap. 1. 

consecutive. But we know, for instance, from Sir R. Murchison*s 
great work on Russia, what wide gaps there are in that country 
between the superimposed formations ; so it is in North America, 
and in many other parts of the world. The most skilful geologist, 
if his attention had been confined exclusively to these large ter- 
ritories, would never have suspected that, during the periods which 
were blank and barren in his own country, great piles of sediment, 
charged with new and peculiar forms of life, had elsewhere been 
accumulated. And if, in each separate territory, hardly any idea 
can be formed of the length of time which has elapsed between the 
consecutive formations, we may infer that this could nowhere be 
ascertained. The frequent and great changes in the mineralogical 
composition of consecutive formations, generally implying great 
changes in the geography of the surrounding lands, whence the 
sediment was derived, accord with the belief of vast intervals of 
time having elapsed between each formation. 

We can, I think, see why the geological formations of each region 
are almost invariably intermittent ; that is, have not followed each 
other in close sequence. Scarcely any fact struck me more when 
examining many hundred miles of the South American coasts, which 
have been upraised several hundred feet within the recent period, 
than the absence of any recent deposits sufficiently extensive to last 
for even a short geological period. Along the whole west coast, 
which is inhabited by a peculiar marine fauna, tertiary beds are so 
poorly developed, that no record of several successive and peculiar 
marine faunas will probably be preserved to a distant age. A little 
reflection will explain why, along the rising coast of the western 
side of South America, no extensive formations with recent or ter- 
tiary remains can anywhere be found, though the supply of sediment 
must for ages have been great, from the enormous degradation of 
the coast-rocks and from muddy streams entering the sea. The 
explanation, no doubt, is, that the littoral and sub-littoral deposits 
are continually worn away, as soon as they are brought up by the 
slow and gradual rising of the land within the grinding action of 
the coast-waves. 

We may, I think, conclude that sediment must be accumulated 
in extremely thick, solid, or extensive masses, in order to. withstand 
the incessant action of the waves, when first upraised and during 
successive oscillations of level, as well as the subsequent subaerial 
degradation. Such thick and extensive accumulations of sediment 
may be formed in two ways ; either in profound depths of the sea, 
in which case the bottom will not be inhabited by so many and 
such varied forms of life, as the more shallow seas ; and the mass 


when upraised will give an imperfect record of the organisms whicb 
existed in the neighbourhood during the period of its accumulation. 
Or, Bediment may be deposited to any thickness and extent over a 
shallow bottom, if it continue slowly to subside. In this latter case, 
as long as the rate of subsidence and the supply of sediment nearly 
balance each other, the sea will remain shallow and favourable for 
many and varied forms, and thus a rich fossiliferous formation, 
thick enough, when upraised, to resist a large amount of denudation, 
may be formed. 

I am convinced that nearly all our ancient formations, which are 
throughout the greater part of their thickness rich in fossils, have 
thus been formed during subsidence. Since publishing my view2 
on this subject in 1845, 1 have watched the progress of Geology, 
and have been surprised to note how author after author, in treat- 
ing of this or that great formation, has come to the conclusion that 
it was accumulated during subsidence. I may add, that the only 
ancient tertiary formation on the west coast of South America, 
which has been bulky enough to resist such degradation as it has 
as yet suffered, but which will hardly last to a distant geological 
age, was deposited during a downward oscillation of level, and thus 
gained considerable thickness. 

All geological facts tell ua plainly that each area has undergone 
numerous slow oscillations of level, and apparently these oscillations 
have affected wide spaces. Consequently, formations rich in fossils 
and sufficiently thick and extensive to resist subsequent degradation 
will have been formed over wide spaces during periods of subsidence, 
but only where the supply of sediment was sufficient to keep the 
sea shallow and to embed and preserve the remains before they had 
time to decay. On the other hand, as long as the bed of the sea 
remains stationary, thick deposits cannot have been accumulated 
in the shallow parts, which are the most favourable to life. Still 
less can this have happened during the alternate periods of elevation ; 
or, to speak more accurately, the beds which were then accumulated 
will generally have been destroyed by being upraised and brought 
within the limits of the coast-action. 

These remarks apply chiefly to littoral and sublittoral deposits. 
In the case of an extensive aud shallow sea, such as that within a 
large part of the Malay Archipelago, where the depth varies from 
30 or 40 to 60 fathoms, a widely extended formation might be 
fonned during a period of elevation, and yet not suffer excessively 
from denudation during its slow upheaval ; but the thickness of tho 
formation could not be great, for owing to the elevatory movement 
it would be less than the depth in which it was formed ; nor would 


the deposit be much consolidated, nor be capped by ovei lying for- 
mations, so that it would run a good chance of being worn away by 
atmospheric degradation and by the action of the sea during sub- 
sequent oscillations of level. It has, however, been suggested by 
Mr. Hopkins, that if one part of the area, after rising and before 
being denuded, subsided, the deposit formed during the rising move- 
ment, though not thick, might afterwards become protected by fresh 
accumulations, and thus be preserved for a long period. 

Mr. Hopkins also expresses his belief that sedimentary beds of 
considerable horizontal extent have rarely been completely destroyed. 
But all geologists, excepting the few who believe that our present 
metamorphic schists and plutonic rocks once formed the primordial 
nucleus of the globe, will admit that these latter rocks have been 
stript of their covering to an enormous extent. For it is scarcely 
possible that such rocks could have been solidified and crystallized 
whilst uncovered ; but if the metamorphic action occurred at pro- 
found depths of the ocean, the former protecting mantle of rock 
may not have been very thick. Admitting tben that gneiss, mica- 
schist, granite, diorite, &c., were once necessarily covered up, how 
can we account for the naked and extensive areas of such rocks in 
many parts of the world, except on the belief that they have sub- 
sequently boen completely denuded of all overlying strata ? That 
such extensive areas do exist cannot be doubted : the granitic region 
of Parime is described by Humboldt as being at least nineteen times 
as large as Switzerland. South of the Amazon, Boue colours an 
area composed of rocks of this nature as equal to that of Spain, 
France, Italy, part of Germany, and the British Islands, all con- 
joined. This region has not been carefully explored, but from the 
concurrent testimony of travellers, the granitic area is very large : 
thus. Von Eschwege gives a detailed section of these rocks, 
stretching from Eio de Janeiro for 260 geographical miles inland in 
a straight line ; and I travelled for 150 miles in another direction, 
and saw nothing but granitic rocks. Numerous specimens, col- 
lected along the whole coast from near Eio Janeiro to the mouth of 
the Plata, a distance of 1100 geographical miles, were examined by 
me, and they all belonged to this class. Inland, along the whole 
northern bank of the Plata I saw, besides modern tertiary beds, only 
one small patch of slightly metamorphosed rock, which alone could 
have formed a part of the original capping of the granitic scries. 
Turning to a well-known region, namely, to the United States and 
Canada, as shown in Professor H. D. Rogers's beautiful map, I have 
estimated the areas by cutting out and weighing the paper, and I 
find that the metamorphic (excluding " the semi-metamorphic ") 


and granitic rocks exceed, in the proportion of 19 to 12'5, the wliole 
of the newer Pala307;oic formations. In many regions the metamor- 
phic and granitic rocks would be fomid much more widely extended 
than they appear to be, if all the sedimentary beds were removed which 
rest unconformably on them, and which could not have formed part 
of the original mantle under which they were crystallized. Hence 
it is probable that in some parts of the world whole formations have 
been completely denuded, with not a wreck left behind. 

One remark is here worth a passing notice. During periods of 
elevation the area of the land and of the adjoining shoal parts of the 
sea will be increased, and new stations will often be formed ; — all 
circumstances favourable, as previously explained, for the formation 
of new varieties and species ; but during such periods there will 
generally be a blank in the geological record. On the other hand, 
during subsidence, the inhabited area and number of inhabitants 
will decrease (excepting on the shores of a continent when first 
broken up into an archipelago), and consequently during subsidence, 
though there will be much extinction, few new varieties or species 
will be formed ; and it is during these very periods of subsidence, 
that the deposits which are richest in fossils have been accumulated. 

On the Absence of Numerous Intermediate Varieties in any Single 


From these several considerations, it cannot be doubted that the 
geological record, viewed as a whole, is extremely imperfect ; but if 
we confine our attention to any one formation, it becomes much 
more difiBcult to understand why we do not therein find closely 
graduated varieties between the allied species which lived at its 
commencement and at its close. Several cases are on record of the 
same species presenting varieties in the upper and lower parts of 
the same formation : thus, Trautschold gives a number of instances 
with Ammonites ; and Hilgendorf has described a most curious case 
of ten graduated forms of Planorbis multiformis in the successive 
beds of a fresh-water formation in Switzerland. Although each 
formation has indisputably required a vast number of years for its 
deposition, several reasons can be given why each should not com- 
monly include a graduated series of links between the species which 
lived at its commencement and close; but I cannot assign due 
proportional weight to the following considerations. 

Although each formation may mark a very long lapse of -j ears, 
each probably is short compared with the period requisite to change 
one species into another. I am aware that two palaeontologists, 
whose opinions are worthy of much deference, namely Bronn and 

T 2 


Woodward, have concluded that the average duration of each for- 
mation is twice or thrice as long as the average duration of specific 
forms. But insuperable diflScultics, as it seems to me, prevent us 
from coming to any just conclusion on this head. When we see a 
species first appearing in the middle of any formation, it would be 
rash in the extreme to infer that it had not elsewhere previously 
existed. So again when we find a species disappearing before the 
last layers have been deposited, it would be equally rash to suppose 
that it then became extinct. We forget how small the area of 
Europe is compared with the rest of the world ; nor have the 
several stages of the same formation throughout Europe been cor- 
related with perfect accuracy. 

We may safely infer that with marine animals of all kinds there 
has been a large amount of migration due to climatal and other 
changes ; and when we see a species first appearing in any forma- 
tion, the probability is that it only then first immigrated into that 
area. It is well known, for instance, that several species appeared 
somewhat earlier in the palaeozoic beds of North America than in 
those of Europe ; time having apparently been required for their 
migration from the American to the European seas. In examining 
the latest deposits in various quarters of the world, it has every- 
where been noted, that some few still existing species are common 
in the deposit, but have become extinct in the immediately sur- 
rounding sea ; or, conversely, that some are now abundant in the 
neighbouring sea, but are rare or absent in this particular deposit. 
It is an excellent lesson to reflect on the ascertained amount of 
migration of the inhabitants of Europe during the glacial epoch, 
which forms only a part of one whole geological period ; and like- 
wise to reflect on the changes of level, on the extreme change of 
climate, and on the great lapse of time, all included within this 
same glacial period. Yet it may be doubted whether, in any 
quarter of the world, sedimentary deposits, including fossil remains, 
have gone on accumulating within the same area during the whole 
of this period. It is not, for instance, probable that sediment was 
deposited during the whole of the glacial period near the mouth of 
the Mississippi, within that limit of depth at which marine animals 
can best flourish: for we know that great geographical changes 
occuiTcd in other parts of America during this space of time. When 
\uch beds as were deposited in shallow water near the mouth of the 
iilississippi during some part of the glacial period shall have been 
upraised, organic remains will probably first appear and disappear 
pt different levels, owing to the migrations of species and to geo- 
graphical changes. And in the distant future, a geologist, examining 


these beds, would be tempted to conclude that the average duration 
of life of the embedded fossils had been less than that of the glacial 
period, instead of having been really far greater, that is, extending 
from before the glacial epoch to the present day. 

In order to get a perfect gradation between two forms in the upper 
and lower parts of the same formation, the deposit must have gone 
on continuously accumulating during a long period, sufficient for 
the slow process of modification ; hence the deposit must be a very 
thick one ; and the species undergoing change must have lived in 
the same district throughout the whole time. But we have seen 
that a thick formation, fossiliferous throughout its entire thickness, 
can accumulate only during a period of subsidence ; and to keep the 
depth approximately the same, which is necessary that the same 
marine species may live on the same space, the supply of sediment 
must nearly counterbalance the amount of subsidence. But this 
same movement of subsidence will tend to submerge the area 
whence the sediment is derived, and thus diminish the supply, 
whilst the downward movement continues. In fact, this nearly 
exact balancing between the supply of sediment and the amount of 
subsidence is probably a rare contingency ; for it has been observed 
by more than one palaeontologist, that very thick deposits are 
usually barren of organic remains, except near their upper or lower 

It would seem that each separate formation, like the whole pile 
of formations in any country, has generally been intermittent in its 
accumulation. When we see, as is so often the case, a formation 
composed of beds of widely different mineralogical composition, we 
may reasonably suspect that the process of deposition has been 
more or less interrupted. Nor will vhe closest inspection of a for- 
mation give us any idea of the length of time which its deposition 
may have consumed. Many instances could be given of beds only 
a few feet in thickness, representing formations, which are else- 
where thousands of feet in thickness, and which must have required 
an enormous period for their accumulation ; yet no one ignorant of 
this fact would have even suspected the vast lapse of time repre- 
sented by the thinner formation. Many cases could be given of 
the lower beds of a formation having been upraised, denuded, sub- 
merged, and then re-covered by the upper beds of the same forma- 
tion, — facts, show'ng what wide, yet easily overlooked, intervals 
have occurred in its accumulation. In other cases we have the 
plainest evidence in great fossilised trees, still standing upright as 
they grew, of many long intervals of time and changes of level 
during the process of deposition, which would not have been sue- 


pected, had not the trees been preserved : thus Sir C. Lyell and 
Dr. Dawson found carboniferous beds 1400 feet thick in Nova 
Scotia, with ancient root-bearing strata, one above the other at no 
less than sixty-eight different levels. Hence, when the same species 
occurs at the bottom, middle, and top of a formation, the proba- 
bility is that it has not lived on the same spot during the whole 
period of deposition, but has disappeared and reappeared, perhaps 
many times, during the same geological period. Consequently if it 
were to undergo a considerable amount of modification during the 
deposition of any one geological formation, a section would not in- 
clude all the fine intermediate gradations which must on our theory 
have existed, but abrupt, though perhaps slight, changes of form. 

It is all-important to remember that naturalists have no golden 
rule by which to distinguish species and varieties ; they grant 
some little variability to each species, but when they meet with ? 
somewhat greater amount of dilierence between any two forms, 
they rank both as species, unless they are enabled to connect them 
together by the closest intermediate gradations; and this, from 
the reasons just assigned, we can seldom hope to effect in any one 
geological section. Supposing B and C to be two species, and a 
third, A, to be found in an older and underlying bed ; even if A 
were strictly intermediate between B and C, it would simply be 
ranked as a third and distinct species, unless at the same time it 
could be closely connected by intermediate varieties with either one 
or both forms. Nor should it be forgotten, as before explained, 
that A might be the actual progenitor of B and C, and yet would 
not necessarily be strictly intermediate between them in all respects. 
So that we might obtain the parent-species and its several modified 
descendants from the lower anei upper beds of the same formation, 
and unless we obtained numerous transitional gradations, we should 
aot recognise their blood-relationship, and should consequently rank 
them as distinct species. 

It is notorious on what excessively slight differences many palas- 
oiitologists have founded their species ; and they do this the more 
readily if the specimens come from different sub-stages of the same 
formation. Some experienced conchologists are now sinking many 
of the very fine species of D'Orbigny and others into the rank of 
varieties; and on this view we do find the kind of evidence of 
change which on the theory we ought to find. Look again at the 
later tertiary deposits, which include many shells believed by the 
majority of naturalists to be identical with existing species ; but 
some excellent naturalists, as Agassiz and Pictet, maintain that ail 
these tertiary species are specifically distinct, though the distincticn 


is admitted to be very slight ; so that here, unless we believe that 
these eminent naturalists have been misled by their imaginations, 
and that these late tertiary species really present no difference what- 
ever from their living representatives, or unless we admit, in oppo- 
sition to the judgment of most naturalists, that these tertiary species 
are all truly distinct from the recent, we have evidence of the fre- 
quent occurrence of slight modifications of the kind required. If 
we look to rather wider intervals of time, namely, to distinct but 
consecutive stages of the same great formation, we find that the 
embedded fossils, though universally ranked as specifically different, 
yet are far more closely related to each other than are the species 
found in more widely separated formations ; so that here again we 
have undoubted evidence of change in the direction required by the 
theory; but to this latter subject I shall return in the following 

With animals and plants that propagate rapidly and do not 
wander much, there is reason to suspect, as we have formerly seen, 
that their varieties are generally at first local ; and that such local 
varieties do not spread widely and supplant their parent-forms until 
they have been modified and perfected in some considerable degree. 
According to this view, the chance of discovering in a formation in 
any one country all the early stages of transition between any two 
forms, is small, for the successive changes are supposed to have 
been local or confined to some one spot. Most marine animals have 
ii wide range ; and we have seen that with plants it is those which 
have the widest range, that oftenest present varieties ; so that, with 
shells and other marine animals, it is probable that those which 
had the widest range, far exceeding the limits of the known geo- 
logical formations of Europe, have oftenest given rise, first to local 
varieties and ultimately to new species; and this again would 
greatly lessen the chance of our being able to trace the stages of 
transition in any one geological formation. 

It is a more important consideration, leading to the same result, 
as lately insisted on by Dr. Falconer, namely, that the period during 
which each species underwent modification, though long as measured 
by years, was probably short in comparison with that during which 
it remained without undergoing any change. 

It should not be forgotten, that at the present day, with perfect 
specimens for examination, two forms can seldom be connected by 
intermediate varieties, and thus proved to be the same species, 
until many specimens are collected from many places; and with 
fossil species this can rarely be done. We shall, perhaps, best per- 
ceive th-3 improbability of our being enabled to connect species 


by numerous, fine, intermediate, fossil links, by asking ourselves 
whether, for instance, geologists at some future period will be able 
to prove that our different breeds of cattle, sheep, horses, and dogs 
are descended from a single stock or from several aboriginal stocks ; 
or, again, whetlicr certain sea-shells inhabiting the shores of North 
America, which are ranked by some conchologists as distinct species 
from their European representatives, and by other conchologists as 
only varieties, are really varieties, or are, as it is called, specifically 
distinct. This could be effected by the future geologist only by 
his discovering in a fossil state numerous intermediate gradations ; 
and such success is improbable in the highest degree. 

It has been asserted over and over again, by writers who belie\'e 
in the immutability of species, that geology yields no linking forms. 
This assertion, as we shall see in the next chaptei, is certainly 
erroneous. As Sir J. Lubbock has remarked, " Every species is a 
link between other allied forms." If we take a genus having a score 
of species, recent and extinct, and destroy four-fifths of them, no 
one doubts that the remainder will stand much more distinct from 
each other. If the extreme forms in the genus happen to have 
been thus destroyed, the genus itself will stand more distinct from 
other allied genera. What geological research has not revealed, is 
the former existence of infinitel}'' numerous gradations, as fine as 
existing varieties, connecting together nearly all existing and extinct 
species. But this ought not to be expected ; yet this lias been 
repeatedly advanced as a most serious objection against my views. 

It may be worth while to sum up the foregoing remarks on the 
causes of ^he imperfection of the geological record under an imagi- 
nary illuF^ration. The Malay Archipelago is about the size of 
Europe from the North Cape to the Mediterranean, and from Britain 
to Eussia ; and therefore equals all the geological formations which 
have been examined with any accuracy, excepting those of tho 
United States of America. I fully agree with Mr. God win- Austen, 
that the present condition of the Malay Archipelago, with its nume- 
rous large islands separated by wide and shallow seas, probably 
represents the former state of Europe, whilst most of our formations 
were accumulating. The Malay Archipelago is one of the richest 
regions in organic beings ; yet if all the species were to be collected 
which have ever lived there, how imperfectly would they represent 
the natural history of the world ! 

But we have every reason to believe that the terrestrial pro- 
ductions of the archipelago would be preserved in an extremely 
imperfect manner in the formations which we suppose to be thera 
accumulating. Not many of the strictly littoral animals, or of 


those which Uved oa naked submarine rocks, would be embedded ; 
tind those embedded in gravel or sand would not endure to a distant 
epoch. Wherever sediment did not accumulate on the bed of the 
eea, or where it did not accumulate at a sufficient rate to protect 
organic bodies from decay, no remains could be preserved. 

Formations rich in fossils of many kinds, and of thickness 
sufficient to last to an age as distant in futurity as the secondary 
formations lie in the past, would generally be formed in the archi- 
pelago only during periods of subsidence. These periods ot 
subsidence would be separated from each other by immense inter- 
vals of time, during which the area would be either stationary or 
rising ; whilst rising, the fossiliferous formations on the steeper 
shores would be destroyed, almost as soon as accumulated, by 
the incessant coast-action, as we now see on the shores of Soiitli 
America. Even throughout the extensive and shallow seas within 
the archipelago, sedimentary beds could hardly be accumulated of 
great thickness during the periods of elevation, or become capped 
and protected by subsequent deposits, so as to have a good chance 
of enduring to a very distant future. During the periods of sub- 
sidence, there would probably be much extinction of life ; during 
the periods of elevation, there would be much variation, but the 
geological record would then be less perfect. 

It may be doubted whether the duration of any one great period 
of subsidence over the whole or part of the archipelago, together 
with a contemporaneous accumulation of sediment, would exceed the 
average duration of the same specific forms ; and these contingen- 
cies are indispensable for the preservation of all the transitional 
gradations between any two or more species. If such gradations 
were not all fully preserved, transitional varieties would merely 
appear as so many new, though closely allied species. It is also 
probable that each great period of subsidence would be interrupted 
by oscillations of level, and that slight climatal changes would inter- 
vene during such lengthy periods ; and in these cases the inhabitants 
of the archipelago would migrate, and no closely consecutive record 
of their modifications could be preserved in any one formation. 

Very many of the marine inhabitants of the archipelago now 
i*ange thousands of miles beyond its confines ; and analogy plainly 
leads to the belief that it would be chiefly these far-ranging species, 
though only some of them, which would oftenest produce new 
varieties ; and the vaneties would at first be local or confined 
to one place, but if possessed of any decided advantage, or whes 
further modified and improved, they would slowly spread and 
supplant their parent-forms When such varieties returned t4i 


their ancient homes, as tliey would differ from their former state 
in a nearly uniform, though perhaps extremely slight degree, and 
as they would be found embedded in slightly different sub-stages of 
tho same formation, they would, according to the principles followed 
by many palaeontologists, be ranked as new and distinct species. 

If then there be some degree of truth in these remarks, we have 
no right to expect to find, in our geological formations, an infinite 
number of those fine transitional forms which, on our theory, have 
connected all the past and present species of the same group into 
one long and branching chain of life. We ought only to look for a 
few links, and such assuredly we do find — some more distantly, 
some more closely, related to each other ; and these links, let them 
be ever so close, if found in different stages of the same formation, 
would, by many palaeontologists, be ranked as distinct species. 
But I do not pretend that I should ever have suspected how poor 
was the record in the best preserved geological sections, had not the 
absence of innumerable transitional links between the species which 
lived at the commencement and close of each formation, pressed so 
hardly on my theory. 

On the sudden Appearance of whole Groups of allied Species. 

The abrupt manner in which whole groups of species suddenly 
appear in certain formations, has been urged by several palaeontolo- 
gists — for instance, by Agassiz, Pictet, and Sedgwick — as a fatal 
objection to the belief in the transmutation of species. If numerous 
species, belonging to the same genera or families, have really started 
into life at once, the fact would be fatal to the theory of evolution 
through natural selection. For the development by this means of 
a group of forms, all of which are descended from some one progeni- 
tor, must have been an extremely slow process ; and the progenitors 
must have lived long before their modified descendants. But we 
continually overrate the perfection of the geological record, and 
falsely infer, because certain genera or families have not been found 
beneath a certain stage, that they did not exist before that stage. 
In ail cases positive palasontological evidence may be implicitly 
trusted; negative evidence is worthless, as experience has so often 
shown. We continually forget how large the world is, compared 
with the area over which our geological formations have been 
caref'iUy examined; we forget tliat groups of species m.ay else- 
where have long existed, and have slowly multiplied, "before 
they invaded the ancient archipeJagoes of Europe and the United 
States. We do not make due allowance for the intervals of 
time which have elapsed between our consecutive formations, — 


longer perhaps in many cases than the time required for the accu- 
mulation of each formation. These intervals will have given time 
for the multiplication of species from some one parent-form ; and in 
the siiojeeding formation, such groups or species will appear as if 
suddenly created. 

I may here recall a remark formerly made, namely, that it might 
require a long succession of ages to adapt an organism to some new 
and peculiar line of life, for instance, to fly through the air ; and 
consequently that the transitional forms would often long remain 
confined to some one region ; but that, when this adaptation had 
once been effected, and a few species had thus acquired a great 
advantage over other organisms, a comparatively short time would 
be necessary to produce many divergent forms, which would spread 
rapidly and widely throughout the world. Professor Pictet, in his 
excellent Review of this work, in commenting on early transitional 
forms, and taking birds as an illustration, cannot see how the suc- 
cessive modifications of the anterior limbs of a supposed prototype 
could possibly have been of any advantage. But look at the 
penguins of the Southern Ocean; have not th-ese birds their front 
limbs in this precise intermediate state of " neither true arms nor 
true wings"? Yet these birds hold their place victoriously in the 
battle for life ; for they exist in infinite numbers and of many kinds. 
I do not suppose that we here see the real transitional grades through 
which the wings of birds have passed ; but what special difficulty is 
there in believing that it might profit the modified descendants of 
the penguin, first to become enabled to flap along the surface of the 
sea like the logger-headed duck, and ultimately to rise from its 
surface and glide through the air ? 

I will now give a few examples to illustrate the foregoing remarks, 
and to show how liable we are to error in supposing that whole 
groups of species have suddenly been produced. Even in so short 
an interval as that between the first and second editions of Pictet'a 
great work on Palaeontology, published in 1844-46 and in 1853-57, 
the conclusions on the first appearance and disappearance of several 
groups of animals have been considerably modified ; and a third 
edition would require still further changes. I may recall the well- 
known fact that in geological treatises, published not many years 
ago, mammals were always spoken of as having abruptly come in 
at the commencement of the tertiary series. And now one of 
the richest known accumulations of fossil mammals belongs to the 
middle of the secondary series ; and true mammals have been dis- 
covered in the new red sandstone at nearly the commencement of 
this great series. Cuvier used to urge that no monkey occurred in 


any tertiary stratum ; but now extinct species have been discovered 
in India, South America, and in Europe, as far back as the miocene 
stage. Had it not been for the rare accident of the preservation of 
footsteps in the new red sandstone of the United States, who would 
have ventured to suppose that no less than at least thirty different 
bird-like animals, some of gigantic size, existed during that period? 
Not a fragment of bone has been discovered in these beds. Not long 
ago, palasontologists maintained that the whole class of birds came 
suddenly into existence during the eocene period ; but now we 
know, on the authority of Professor Owen, that a bird certainly 
lived during the deposition of the upper greensand ; and still more 
recently, that strange bird, the Archeopteryx, with a long lizard- 
like tail, bearing a pair of feathers on each joint, and with its wings 
furnished with two free claws, has been discovered in the oolitic 
slates of Solenhofen. Hardly any recent discovery shows more 
forcibly than this, how little we as yet know of the former inhabi 
tants of the world. 

I may give another instance, which, from having passed under 
my own eyes, has much struck me. In a memoir on Fossil Sessile 
Cirripedes, I stated that, from the large number of existing and 
extinct tertiary species ; from the extraordinary abundance of the 
individuals of many species all over the world, from the Arctic 
regions to the equator, inhabiting various zones of depths from the 
Uj )per tidal limits to 50 fathoms ; from the perfect manner in which 
specimens are preserved in the oldest tertiary beds ; from the case 
with which even a fragment of a valve can be recognised ; from all 
these circumstances, I inferred that, had sessile cirripedes existed 
during the secondary periods, they would certainly have been pre- 
served and discovered ; and as not one species had then been dis- 
covered in beds of this age, I concluded that this great group had 
been suddenly developed at the commencement of the tertiary series. 
This was a sore trouble to me, adding as I then thought one more 
instance of the abrupt appearance of a great group of species. But 
my work had hardly been published, when a skilful palseontologist, 
M. Bosquet, sent me a drawing of a perfect specimen of an unmis- 
takeable sessile cirripede, which he had himself extracted from the 
chalk of Belgium. And, as if to make the case as striking as 
possible, this cirripede was a Chthamaius, a very common, large, 
and ubiquitous genus, of which not one species has as yet been 
found even in any tertiary stratum. Still more recently, a Pyrgoma, 
a member of a distinct sub-family of sessile cirripedes, has been 
discovered by Mr. Woodward in the upper chalk ; so that we now 


have abundant evidence of the existence of this group of animals 
during the secondary period. 

The case most frequently insisted on by palaeontologists of the 
apparently sudden appearance of a whole group of species, is that of 
the teleostean fishes, low down, according to Agassiz, in the Chalk 
period. This group includes the large majority of existing species. 
But certain Jurassic and Triassic forms are now commonly admitted 
to be teleostean ; and even some palaeozoic forms have thus boen 
classed by one high authority. If the teleosteans had really 
appeared suddenly in the northern hemisphere at the commencement 
of the chalk formation, the fact would have been highly remarkable; 
but it would not have formed an insuperable difficulty, unless it 
could likewise have been shown that at the same period the species 
were suddenly and simultaneously developed in other quarters of 
the world. It is almost superfluous to remark that hardly auy 
fossil-fish are known from south of the equator ; and by running 
through Pictet's Palaeontology it will be seen that very few species 
are known from several formations in Europe. Some few families of 
fish now have a confined range ; the teleostean fishes might formerly 
have had a similarly confined range, and after having been largely 
developed in some one sea, have spread widely. Nor have we 
any right to suppose that the seas of the world have always been 
so freely open from south to north as they are at present. Even at 
this day, if the Malay Archipelago were converted into land, the 
tropical parts of the Indian Ocean would form a large and perfectly 
enclosed basin, in which any great group of marine animals might 
be multiplied ; and here they would remain confined, until some of 
the species became adapted to a cooler climate, and were enabled to 
double the Southern capes of Africa or Australia, and thus reach 
other and distant seas. 

From these considerations, from our ignorance of the geology of 
other countries beyond the confines of Europe and the United 
States, and from the revolution in our paleeontological knowledge 
effected by the discoveries of the last dozen years, it seems to me 
to be about as rash to dogmatize on the succession of organic forms 
thioughout the world, as it would be for a naturalist to land for five 
minutes on a barren point in Australia, and then to discuss the 
number and range of its productions. 

On the sudden Appearance of Groups of allied Species in the 
loivest known Fossiliferous Strata. 

There is another and allied difficulty, which is much more serious. 
I ailnde to the manner in which species belonging to several of the 


main divisions of the animal kingdom suddenly appear in tha 
lowest known fossiliferous rocks. Most of the arguments which 
have convinced me that all the existing species of the same group 
are descended from a single progenitor, apply with equal force to 
the earliest known species. For instance, it cannot be doubted that 
all the Cambrian and Silurian trilobites are descended from some 
one crustacean, which must have lived long before the Cambrian 
age, and which probably differed greatly from any known animal. 
Some of the most ancient animals, as the Nautilus, Lingula, &c., do 
not differ much from living species ; and it cannot on our theory 
be supposed, that these old species were the progenitors of all the 
species belonging to the same groups which have subsequently 
appeared, for they are not in any degree intermediate in character. 

Consequently, if the theory be true, it is indisputable that before 
the lowest Cambrian stratum was deposited, long periods elapsed, as 
long as, or probably far longer than, the whole interval from the 
Cambrian age to the present day ; and that during these vast 
periods the world swarmed with living creatures. Here we en- 
counter a formidable objection ; for it seems doubtful whether the 
earth, in a fit state for the habitation of living creatures, has lasted 
long enough. Sir W. Thompson concludes that the consolidation 
of the crust can hardly have occuiTcd less than 20 or more than 
400 million years ago, but probably not less than 98 or more 
than 200 million years. These very wide limits show how doubt- 
ful the data are ; and other elements may have hereafter to be 
introduced into the problem. Mr. Croll estimates that about 
60 million years have elapsed since the Cambrian period, but this, 
judging from the small amount of organic change since the com- 
mencement of the Glacial epoch, appears a very short time for the 
many and great mutations of life, which have certainly occurred 
since the Cambrian formation ; and the previous 140 million years 
can hardly be considered as sufficient for the development of the 
varied forms of life which already existed during the Cambrian 
period. It is, however, probable, as Sir William Thompson insists, 
that the world at a very early period was subjected to more rapid 
and violent changes in its physical conditions than those now 
occurring ; and such changes would have tended to induce changes 
at a corresponding rate in the organisms which then existed. 

To the question why we do not find rich fossiliferous deposits 
belonging to these assumed earliest periods prior to the Cambrian 
system, I can give no satisfactory answer. Several eminent geo- 
logists, ^ith Sir E. Murchison at their head, were until recently 
convinced that we beheld in the organic remains of the lowest 


Silurian stratum the first dawn of life. Other highly competent 
judges, as Lyell and E. Forbes, have disputed this couclusion. 
We should not forget that only a small portion of the world is 
known with accuracy. Not very long ago M. Barrande added 
another and lower stage, abounding with new and peculiar species, 
beneath the then known Silurian system; and now, still lower 
down in the Lower Cambrian formation, Mr. Hicks has found 
in South Wales beds rich in trilobites, and containing various 
molluscs and annelids. The presence of phosphatic nodules and 
bituminous matter, even in some of the lowest azoic rocks, probably 
indicates life at these periods ; and the existence of the Eozoon in 
the Laurentian formation of Canada is generally admitted. Thera 
are three great series of strata beneath the Silurian system in 
Canada, in the lowest of which the Eozoon is found. Sir W. 
Logan states that their " united thickness may possibly far surpass 
" that of all the succeeding rocks, from the base of the palseozoic 
" series to the present time. We are thus carried back to a period 
" so remote, that the appearance of the so-called Primordial fauna 
" (of Barrande) may by some be considered as a comparatively 
" modern event." The Eozoon belongs to the most lowly organised 
of all classes of animals, but is highly organised for its class ; it 
existed in countless numbers, and, as Dr. Dawson has remarked, 
certainly preyed on other minute organic beings, which must have 
lived in great numbers. Thus the words, which I wrote in 1859, 
about the existence of living beings long before the Cambrian 
period, and which are almost the same with those since used by 
Sir W. Logan, have proved true. Nevertheless, the difficulty of 
assigning any good reason for the absence of vast piles of strata 
rich in fossils beneath the Cambrian system is very great. It does 
not seem probable that the most ancient beds have been quite 
worn away by denudation, or that their fossils have been wholly 
obliterated by metamorphic action, for if this had been the case 
we should have found only small remnants of the formations next 
succeeding them in age, and these would always have existed in a 
partially metamorphosed condition. But the descriptions which we 
possess of thCv-Silurian deposits over immense territories in Eussia 
and in North America, do not support the view, that the older a 
formation is, the more invariably it has suffered extreme denudation 
and metamorphism. 

The case at present must remain inexplicable ; and may be truly 
urged as a valid argument against the views here entertained. To 
show that it may hereafter receive some explanation, I will give 
die following hypothesis. From the nature of the organic remai oa 


Vv'hich do not appear to have inhabited profound depths, in the 
several formations of Europe and of the United States ; and from 
the amount of sediment, miles in thickness, of which the formations 
are composed, we may infer that from first to last large islands 
or tracts of land, whence the sediment was derived, occurred in the 
neighbourhood of the now existing continents of Europe and 
North America. This same view has since been maintained by 
Agassiz and others. But we do not know what was the state 
of things in the intervals between the several successive formations ; 
whether Europe and the United States during these intervals 
existed as dry land, or as a submarine surface near land, on which 
sediment was not deposited, or as the bed of an open and unfathom- 
able sea. 

Looking to the existing oceans, which are thrice as extensive as 
the land, we see them studded with many islands ; but hardly one 
truly oceanic island (with the exception of New Zealand, if this 
can be called a truly oceanic island) is as yet known to afford even 
a remnant of any palreozoic or secondary formation. Hence we may 
perhaps infer, that during the palaeozoic and secondary periods, 
neither continents nor continental islands existed where our oceans 
now extend ; for had they existed, palaeozoic and secondary forma- 
tions would in all probability have been accumulated from sediment 
derived from their wear and tear ; and these would have been at 
least partially upheaved by the oscillations of level, which must 
have intervened during these enormously long periods. If then we 
may infer anything from these facts, we may infer that, where our 
oceans now extend, oceans have extended from the remotest period 
of which we have any record ; and on the other hand, that where 
continents now exist, large tracts of land have existed, subjected no 
doubt to great oscillations of level, since the Cambrian period. The 
coloured map appended to my volume on Coral Reefs, led me to 
conclude that the great oceans are still mainly areas of subsidence, 
the great archipelagoes still areas of oscillations of level, and the 
continents areas of elevation. But we have no reason to assume 
that things have thus remained from the beginning of the world. 
Our continents seem to have been formed by a preponderance, during 
many oscillations of level, of the force of elevation ; but may not 
the areas of preponderant movement have changed in the lapse of 
ages ? At a period long antecedent to the Cambrian epoch, con- 
1 kients may have existed where oceans are now spread out ; and 
^lear and open oceans may have existed where our continents now 
siand. Nor should we be justified in assuming that if, for instance 
tuc bed of the Pacific Ocean were now converted into a continent 


we should there find sedimentary formations in a recognisable 
condition older than the CamLrian strata, supposing such to have 
been formerly deposited ; for it might well happen that strata 
which had g'ibsided some milea nearer to the centre of the earth, 
and which had been pressed on by an enormous weight of superin- 
cumbent water, might have undergone far more metamorphic action 
than strata which have always remained nearer to the surface. 
The immense areas in some parts of the world, for instance in 
South America, of naked metamorphic rocks, which must have been 
heated under great pressure, have always seemed to me to require 
some special explanation ; and we may perhaps believe that we see 
in these large areas, the many formations long anterior to the 
Cambrian epoch in a completely metamorphosed and denuded 

The several difficulties here discussed, namely — that, though we 
find in our geological formations many links betwe-en the species 
which now exist and which formerly existed, we do not find 
infinitely numerous fine transitional forms closely joining them all 
together ; — the sudden manner in which several groups of species 
first appear in our European formations ; — the almost entire absence, 
as at present known, of formations rich in fossils beneath the 
Cambrian strata, — are all undoubtedly of the most serious nature. 
We see this in the fact that the most eminent paleeontologists, 
namely, Cuvier, Agassiz, Barrande, Pictet, Falconer, E. Forbes, &c., 
and ail our greatest geologists, as Lyell, Murchison, Sedgwick, 
&c., have unanimously, often vehemently, maintained the immu- 
tability of species. But Sir Charles Lyell now gives the support of 
his high authority to the opposite side ; and most geologists and 
palaeontologists are much shaken in their former belief. Those 
Avho believe that the geological record is in any degree perfect, will 
undoubtedly at once reject the theory. For my part, following out 
Lyell's metaphor,!! look at the geological record as a history of the 
world imperfectly kept, and written in a changing dialect; of this 
history we possess the last volume alone, relating only to two or 
three countries. Of this volume, only here and there a short 
chapter has been preserved ; and of each page, only here and there 
a few lines. Each word of the slowly-changing language, more or 
less different in the successive chapters, may represent the forms of 
life, which are entombed in our consecutive formations, and which 
falsely appear to us to have been abruptly introduced. On this 
view, the difficulties above discussed are greatly diminished, or even 




On the Geological Succession of Organic Beings. 

On the slow and successive appearance of new species — On their differeiit 
rates of change — Species once lost do not reappear — Groups of species 
follow the same general rules in their appearance and disappearance as 
do single species — On extinction — On simultaneous changes in the 
forms of life throughout the world — On the affinities of extinct species 
to each other and to living species — On the state of development of 
ancient forms — On the succession of the same types within the same 
areas — Summary of preceding and present chapter. 

Let us now see w],ietlier the several facts and laws relating to the 
geological succession of organic beings accord best with the common 
view of the immutability of species, or with that of their slow and 
gradual modification, through variation and natural selection. 

New species have appeared very slowly, one after another, both 
on the land and in the waters. Lyell has shown that it is hardly 
possible to resist the evidence on this head in the case ol the several 
tertiary stages ; and every year tends to fill up the blanks between 
the stages, and to make the proportion between the lost and exist- 
ing forms more gradual. In some of the most recent beds, though 
undoubtedly of high antiquity if measured by years, only one or 
two species are extinct, and only one or two are new, having 
appeared there for the first time, either locally, or, as far as we 
know, on the face of the earth. The secondary formations are 
more broken ; but, as Bronn has remarked, neither the appearance 
nor disappearance of the many species embedded in each formation 
has been simultaneous. 

Species belonging to different genera and classes have not changed 
at the same rate, or in the same degree. In the older tertiary 
beds a few living shells may still be found in the midst of a mul- 
titude of extinct forms. Falconer has given a striking instance 
of a similar fact, for an existing crocodile is associated with many 
losu mammals and reptiles in the sub-Himalayan deposits. The 
Silurian Lingula differs but little from the living species of this 
genus ; whereas most of the other Silurian Molluscs and all the 
Crustaceans have changed greatly. The productions of the land 


seem to have changed at a quicker rate than those of the sea, of 
which a striking instance has been observed in Switzerland. There 
is some reason to believe that organisms high in the scale, change 
more quickly than those that are low : though there are exceptions 
to this rule. The amount of organic change, as Pictet has remarked, 
is not the same in each successive so-called formation. Yet if we 
compare any but the most closely related formations, all the species 
will be found to have undergone some change. When a species 
has once disappeared from the face of the earth, we have no reason to 
believe that the same identical form ever reappears. The strongest 
apparent exception to this latter rule is that of the so-called 
" colonies " of M. Barrande, which intrude for a period in the midst of 
an older formation, and then allow the pre-existing fauna to reappear ; 
but Lyell's explanation, namely, that it is a case of temporary 
migration from a distinct geographical province, seems satisfactory. 
These several facts accord well with our theory, which includea 
no fixed law of development, causing all the inhabitants of an area 
to change abruptly, or simultaneously, or to an equal degree. The 
process of modification must be slow, and will generally affect only 
a few species at the same time ; for the variability of each species 
is independent of that of all others. Whether such variations or 
individual differences as may arise will be accumulated through 
natural selection in a greater or less degree, thus causing a greater 
or less amount of permanent modification, will depend on many 
complex contingencies — on the variations being of a beneficial 
nature, on the freedom of intercrossing, on the slowly changing 
physical conditions of the country, on the immigration of new 
colonists, and on the nature of the other inhabitants with which 
the varying species come into competition. Hence it is by no 
means surprising that one species should retain the same identical 
form much longer than others ; or, if changing, should change in a 
less degree. We find similar relations between the existing inha- 
bitants of distinct countries; for instance, the land-shells and 
coleopterous insects of Madeira have come to differ considerably 
from their nearest allies on the continent of Europe, whereas the 
marine shells and birds have remained unaltered. We can perhaps 
understand the apparently quicker rate of change in terrestrial 
and in more highly organised productions compared with marine 
and lower productions, by the more complex relations of the higher 
beings to their organic and inorganic conditions of life, as explained 
in a fonner chapter. When many of the inhabitants of any area 
have l^ecome modified and improved, we can understand, on the 
principle of competition, anri from the all-important relations of 

u 2 


organism to organism in the struggle for life, that any form which 
did not become in some degree modified and improved, would be 
liable to extermination. Hence we see why all the species in the 
same region do at last, if we look to long enough intervals of time, 
become modified, for otherwise they would become extinct. 

In members of the same class the average amount of change, 
during long and equal periods of time, may, perhaps, be nearly the 
same; but as the accumulation of enduring formations, rich in 
/^ToSsils, depends on great masses of sediment being deposited oq 
; subsiding areas, our formations have been almost necessarily accu- 
mulated at wide and irregularly intermittent intervals of time ; 
consequently the amount of organic change exhibited by the fossils 
embedded in consecutive formations is not equal. Each formation^ 
on this view, does not mark a new and complete act of creation, 
but only an occasional scene, taken almost at hazard^^ an ever 
slowly changing_dram3,- t "^ 

We can clearly understand why a species when once lost should 
never reappear, even if the very same conditions of life, organic and 
inorganic, phould recur. For though the offspring of one species 
might be adapted (and no doubt this has occurred in innumerable 
instances) to fill the place of another species in the economy of 
nature, and thus supplant it ; yet the two forms — the old and the 
new — would not be identically the same ; for both would almost 
certainly inherit different characters from their distinct progenitors ; 
and organisms already differing would vary in a different manner. 
For instance, it is possible, if all our fantail pigeons were destroyed, 
that fanciers might make a new breed hardly distinguishable from 
the present breed ; but if the parent rock-pigeon were likewise 
destroyed, and under nature we have every reason to believe that 
parent-forms are generally supplanted and exterminated by their 
improved offspring, it is incredible that a fantail, identical with the 
existing breed, could be raised from any other species of pigeon, or 
even from any other well-established race of the domestic pigeon, 
for the successive variations would almost certainly be in some 
degree different, and the newly-formed variety would probably 
inherit from its progenitor some characteristic differences. 

Groups of species, that is, genera and families, follow the same 
general rules in their appearance and disappearance as do single 
species, changing more or less quickly, and in a greater or lesser 
degree. A group, when it has once disappeared, never reappears ; 
that is, its existence, as long as it lasts, is continuous. I am 
aware that there are some apparent exceptions to this rule, but the 
exceptions are surprisingly few, so few that E. Forbes, Pictet, and 


Woodw ard (though all strongly opposed to such views as I maintain) 
admit it^ truth ; and the rule strictly accords with the theory. For 
all the sj^ecies of the same group, however long it may have lasted, 
are the modified descendants, one from the other, and all from a 
common progenitor. In the genus Lingula, for instance, the species " 
which have successively appeared at all ages must have been con- 
nected by an unbroken series of generations, from the lowest 

Silurian stratum to the present day. 

7We~liave seen in the last chapter that whole groups of species 
sbmetimes falsely appear to have been abruptly developed ; and ] 
I have attempted to give an explanation of this fact, which if true— > 
mmld be fatal to my views. But such cases are certainly excei)- 
tional ; the general rule being a gradual increase in number, until 
the group reaches its maximum, and then, sooner or later, a gradual 
decrease. If the number of the species included within a genus, 
or the number of the genera within a family, be represented by a 
vertical line of varying thickness, ascending through the successive 
geological formations in which the species are found, the line will 
sometimes falsely appear to begin at its lower end, not in a sharp 
point, but abruptly ; it then gradually thickens upwards, often 
keeping of equal thickness for a space, and ultimately thins out in 
the upper beds, marking the decrease and final extinction of the 
species. This gradual increase in number of the species of a group 
is strictly conformable with the theory, for the species of the same 
genus, and the genera of the same family, can increase only slowly 
and progressively ; the process of modification and the production 
of a number of allied forms necessarily being a slow and gradual 
process, — one species first giving rise to two or three varieties, these 
being slowly converted into species, which in their turn produce by 
equally slow steps other varieties and species, and so on, like the 
branching of a great tree from a single stem, till the group becomes 

On Extinction. 

We have as yet spoken only incidentally of the disappearance of 
Jspecies and of groups of species. On the theory of natural selection, 
the extinction of old forms and the production of new and improved 
forms are intimately connected together. The old notion of all the 
inhabitants of the earth having been swept away by catastrophes 
at successive periods is very generally given up, even by those 
geologists, as Elie de Beaumont, Murchison, Barrande, &c., whose 
general views would n&turally lead them to this conclusion. On 
the contraiy, we have every reason to believe, from the study of the 

State Hibl"?,^:.- 

294 EXTINCTION. [Chap. XL 

tertiary formations, that species and groups of species gradually 
disappear, one after another, first from one spot, then from another, 
and finally from the world. In some few cases, however, as by the 
breaking of an isthmus and the consequent irruption of a multitude 
of new inhabitants into an adjoining sea, or by the final subsidence 
of an island, the process of extinction may have been rapid. Both 
single species and whole groups of species last for very unequal 
periods; some groups, as we have seen, have endured from the 
earliest known dawn of life to the present day ; some have dis- 
appeared before the close of the palaeozoic period. No fixed law 
seems to determine the length of time during which any single 
species or any single genus endures. There is reason to believe that 
the extinction of a whole group of species is generally a slower pro- 
cess than their production : if their appearance and disappearance- 
be represented, as before, by a vertical line of varying thickness 
tlie line is found to taper more gradually at its upper end, which 
marks the progress of extermination, than at its lower end, which 
marks the first appearance and the early increase in number of the 
species. In some cases, however, the extermination of whole 
groups, as of ammonites, towards the close of the secondary period, 
has been wonderfully sudden. 

The extinction of species has been involved in the most gratuitous' 
mystery. Some authors have even supposed that, as the individual 
has a definite length of life, so have species a definite duration. No- 
one can have marvelled more than I have done at the extinction of 
species. When I found in La Plata the tooth of a horse embedded 
with the remains of Mastodon, Megatherium, Toxodon, and other 
extinct monsters, which all co-existed with still living shells at a 
very late geological period, I was filled with astonishment; for^ 
seeing that the horse, since its introduction by the Spaniards into 
South America, has run wild over the whole country and has 
increased in numbers at an unparalleled rate, I asked myself what 
could so recently have exterminated the former horse under con- 
ditions of life apparently so favourable. But my astonishment was 
groundless. Professor Owen soon perceived that the tooth, though 
so like that of the existing horse, belonged to an extinct species. 
Had this horse been still living, but in some degree rare, no naturalist 
would have felt the least surprise at its rarity ; for rarity is the 
attribute of a vast number of species of all classes, in all countries. 
If we ask ourselves why this or that species is rare, we answer- 
that something is unfavourable in its conditions of life ; but what 
that something is, we can hardly ever tell. On the supposition of 
the fossil horse still existing as a rare species, we might have felt 


certain, from the analogy of all other mammals, even of the slow- 
breeding elephant, and from the history of the natm*alisation of the 
domestic horse in South America, that under more favourable con- 
ditions ir, would in a very few years have stocked the whole 
continent. But we could not have told what the unfavourable con- 
ditions were which checked its increase, whether some one or several 
contingencies, and at what period of the horse's life, and in what 
degree, they severally acted. If the conditions had gone on, how- 
ever slowly, becoming less and less favourable, we assuredly should 
not have perceived the fact, yet the fossil horse would certainly 
have become rarer and rarer, and finally extinct ; — its place being 
seized on by some more successful competitor. 

It is most difficult always to remember that the increase of every 
creature is constantly being checked by unperceived hostile agencies j 
and that these same unperceived agencies are amply sufficient to 
cause rarity, and finally extinction. So little is this subject under- 
stood, that I have heard surprise repeatedl}^ expressed at such great 
monsters as the Mastodon and the more ancient Dinosaurians having 
become extinct; as if mere bodily strength gave victory in the 
battle of life. Mere size, on the contrary, would in some cases 
determine, as has been remarked by Owen, quicker extermination 
from the greater amount of requisite food. Before man inhabited 
India or Africa, some cause must have checked the continued 
increase of the existing elephant. A highly capable judge. Dr. 
Falconer, believes that it is chiefly insects which, from incessantly 
harassing and weakening the elephant in India, check its increase ; 
and this was Bruce's conclusion with respect to the African elephant 
in Abyssinia. It is certain that insects and blood-sucking bats 
determine the existence of the larger naturalised quadrupeds ia 
several parts of S. America. 

We see in many cases in the more recent tertiar}^ formations, 
that rarity precedes extinction ; and we know that this has been 
the progress of events with those animals which have been exter- 
minated, either locally or wholly, through man's agency. I may 
repeat what I published in 1845, namely, that to admit that species 
generally become rare before they become extinct — to feel no sur- 
prise at the rarity of a species, and yet to marvel greatly when the 
species ceases to exist, is much the same as to admit that sickness 
in the individual is the forerunner of death — to feel no surprise 
at sickness, but, when the sick man dies, to wonder and to suspect 
that he died by some deed of violence. 

The theory of natural selection is grounded on the belief that 
each new variety and ultimately each new species, is produced and 

296 EXTINCTION. [Chap. XI. 

maintained by having some advantage over those with which it 
comes into competition ; and the consequent extinction of the less- 
favoured forms ahuost inevitably follows. It is the same with our 
domestic productions ; when a new and slightly improved variety 
has been raised, it at first supplants the less improved varieties in 
the same neighbourhood ; when much improved it is transported 
far and near, like our short-horn cattle, and takes the place of other 
breeds in other countries. Thus the appearance of new forms and 
the disappearance of old forms, both those naturally and those arti- 
ficially produced, are bound together. In flourishing groups, the 
number of new specific forms which have been produced within a 
given time has at some periods probably been greater than the 
number of the old specific forms which have been exterminated ; 
but we know that species have not gone on indefinitely increasing, 
at least during the later geological epochs, so that, looking to later 
times, we may believe that the production of new forms has caused 
the extinction of about the same number of old forms. 

The competition will generally be most severe, as formerly ex- 
plained and illustrated by examples, between the forms which are 
most like each other in all respects. Hence the improved and 
modified descendants of a species will generally cause the extermi- 
nation of the parent-species ; and if many new forms have been 
developed from any one species, the nearest allies of that species, 
i.e. the species of the same genus, will be the most liable to extermi- 
nation. Thus, as I believe, a number of new species descended 
from one species, that is a new genus, comes to supplant an old 
genus, belonging to the same family. But it must often have 
happened that a new species belonging to some one group has seized 
on the place occupied by a species belonging to a distinct group, 
and thus have caused its extermination. If many allied forms be 
developed from the successful intruder, many will have to yield 
their places ; and it will generally be the allied forms, which will 
suffer from some inherited inferiority in common. But whether it 
be species belonging to the same or to a distinct class, which have 
yielded their places to other modified and improved species, a few 
of the sufferers may often be preserved for a long time, from being 
fitted to some peculiar line of life, or from inhabiting some distant 
and isolated station, where they will have escaped severe competi- 
tion. For instance, some species of Trigonia, a great genus of shells 
in the secondary formations, survive in the Australian seas ; and a few 
members of the great and almost extinct group of Ganoid fishes still 
inhabit our fresh waters. Therefore the utter extinction of a group 
is generally, as we have seen, a slower process than its production. 

Chap. XL] EXTINCTION. 297 

With respect to the apparently sudden externTiination of whole 
families or orders, as cf Trilobites at the close of the palaeozoic 
period and of Ammonites at the close of the secondary period, we 
mnst remember what has been already said on the probable wide 
intervals of time between our consecutive formations ; and in these 
intervals there may have been much slow extermination. More- 
over, when, by sudden immigration or by unusually rapid develop- 
ment, many species of a new group have taken possession of an 
area, many of the older species will have been exterminated in a 
correspondingly rapid manner; and the forms which thus yield 
their places will commonly be allied, for they will partake of the 
same inferiority in common. 

Thus, as it seems to me, the manner in which single species and 
whole groups of species become extinct accords well with the theory 
of natural selection. We need not marvel at extinction ; if we 
must marvel, let it be at our own presumption in imagining for a 
moment that we understand the many complex contingencies on 
which the existence of each species depends. If we forget for an 
instant, that each species tends to increase inordinately, and that 
some check is always in action, yet seldom perceived by us, the 
whole economy of nature will be utterly obscured. Whenever we 
can precisely say why this species is more abundant in individuals 
than that ; why this species and not another can be naturalised in 
a given country ; then, and not until then, we may justly feel sur- 
prise why we cannot account for the extinction of any particulai 
species or group of species. 

On the Forms of Life changing almost simultaneously throughout 

the World. 

Scarcely any palseontological discovery is more striking than the 
fact, that the forms of life change almost simultaneously throughout 
tk'C world. Thus our European Chalk formation can be recognised 
in many distant regions, under the most different climates, where 
not a fragment of the mineral chalk itself can be found ; namely, 
in North America, in equatorial South America, in ^J'ierra del 
Fuego, at the Cape of Good Hope, and in the peninsula of India. 
For at these distant points, the organic remains in certain beds pre- 
sent an unmistakeable resemblance to those of the Chalk. It is 
not that the same species are met with ; for in some cases not one 
species is identically the same, but they bekng to the same fami- 
lies, genera, and sections of genera, and sometimes are similarly 
characterised in such trifling points as mere superficial sculpture. 
Moreover, otlier forms, which are not found in the Chalk of Europe 


but whiich occur in the formations either above or below, (xjcur in 
the same order at these distant points of the world. In the several 
successive palaeozoic formations of Eussia, Western Europe, and 
North America, a similar parallelism in the forms of life has been 
observed by several authors : so it i.s, according to Lyell, with the 
European and North American tertiary deposits. Even if the few 
lossil species which are common to the Old and New Worlds were 
kept wholly out of view, the general parallelism in the successive 
forms of life, in the palaeozoic and tertiary stages, would still be 
manifest, and the several formations could be easily correlated. 

These observations, however, relate to the marine inhabitants of 
the world : we have not sufficient data to judge whether the pro- 
ductions of the land and of fresh water at distant points change in 
the same parallel manner. We may doubt whether they have thus 
changed : if the Megatherium, Mylodon, Macrauchenia, and Toxo- 
don had been brought to Europe from La Plata, without any in- 
formation in regard to their geological position, no one would have 
suspected that they had co-existed with sea-shells all still living ; 
but as these anomalous monsters co-existed with the Mastodon and 
Horse, it might at least have been inferred that they had lived 
during one of the later tertiary stages. 

When the marine forms of life are spoken of as having changed 
simultaneously throughout the world, it must not be supposed that 
this expression relates to the same year, or to the same century > 
or even that it has a very strict geological sense ; for if all the 
marine animals now living in Europe, and all those that lived 
in Europe during the pleistocene period (a very remote period as 
measured by years, including the whole glacial epoch) were com- 
pared with those now existing in South America or in Australia, 
the most skilful naturalist would hardly be able to say whether 
the present or the pleistocene inhabitants of Europe resembled most 
closely those of the southern hemisphere. So, again, several highly 
competent observers maintain that the existing productions of the 
United States are more closely related to those which lived in 
Europe during certain late tertiary stages, than to the present 
inhabitants of Europe ; and if this be so, it is evident that fossili- 
ferous beds now deposited on the shores of North America would 
hereafter be liable to be classed with somewhat older European 
beds. Nevertheless, looking to a remotely future epoch, there can 
be little doubt that all the more modern raarine formations, namely,, 
the upper pliocene, the pleistocene and strictly modern beds, of 
Europe, North and South America, and Australia, from containing: 
fossil remains in some degree allied, and from not including those 


forms which are found only in the older underlying deposits, would 
he correctly ranked as simultaneous in a geological sense. 

The fact of the forms of life changing simultaneously, in the 
above large sense, at distant parts of the world, has greatly struck 
those admirable observers, MM. de Verneuil and d'Archiac. After 
referring to the parallelism of the palasozoic forms of life in various 
parts of Europe, they add, " If, struck by this strange sequence, we 
" turn our attention to North America, and there discover a series 
" of analogous phenomena, it will appear certain that all these modi- 
" fications of species, their extinction, and the introduction of new 
" ones, cannot be owing to mere changes in marine currents or other 
"causes more or less local and temporary, but depend on genera. 
" laws which govern the whole animal kingdom." M. Barrando 
has made forcible remarks to precisely the same effect. It is, indeed, 
quite futile to look to changes of currents, climate, or other physical 
conditions, as the cause of these great mutations in the forms of life 
throughout the world, under the most different climates. We must, 
as Barrande has remarked, look to some special law. We shall see 
this more clearly when we treat of the present distribution of organic 
beings, and find how slight is the relation between the physical 
conditions of various countries and the nature of their inhabitants. 

This great fact of the parallel succession of the fonns of life 
throughout the world, is explicable on the theory of natural selec- 
tion. New species are formed by having some advantage over 
older forms ; and the forms, which are already dominant, or have 
some advantage over the other forms in their own country, give 
birth to the greatest number of new varieties or incipient species. 
We have distinct evidence on this head, in the plants which are 
dominant, that is, which are commonest and most widely diflused, 
producing the greatest number of new varieties. It is also natural 
that the dominant, varying, and far-spreading species, which have 
already invaded to a certain extent the territories of other species, 
should be those which would have the best chance of spreading still 
further, and of giving rise in new countries to other new varieties 
and species. The process of diffusion would often be very slow, 
depending on climatal and geographical changes, on strange acci- 
dents, and on the gradual acclimatisation of new species to the 
various climates through which they might have to pass, but in 
the course of time the dominant forms would generally succeed in 
spreading and would ultimately prevail. The diffusion would, it k 
probable, be slower with the terrestrial inhabitants of distinct con 
linents than with the marine inhabitants of the continuous sea. 
We might therefore expect to find, as we do find, a less strict degree 


of parallelism in the succession of the productions of the land than 
with those of the sea. 

Thus, as it seems to me, the parallel, and, taken in a large sense, 
simultaneous, succesi^ion of the same forms of life throughout the 
world, accords well \vith the principle of new species having been 
formed by dominant species spreading widely and varying ; the new 
species thus produced being themselves dominant, owing to their 
having had some advantage over their already dominant parents, as 
well as over other species, and again spreading, varying, and j)ro- 
ducing new forms. The old forms which are beaten and which 
yield their places to the new and victorious forms, will generally'' bt 
allied in groups, from inheriting some inferiority in common ; and 
therefore, as new and improved groups spread throughout the world, 
old groups disappear from the world ; and the succession of form? 
everywhere tends to correspond both in their first appearance and 
final disappearance. 

There is one other remark connected with this subject worth 
making. I have given my reasons for believing that most of our 
great formations, rich in fossils, were deposited during periods of 
subsidence ; and that blank intervals of vast duration, as far as 
fossils are concerned, occurred during the periods when the bed of 
the sea was either stationary or rising, and likewise when sediment 
was not thrown down quickly enough to embed and pi-eserve organic 
remains. During these long and blank intervals I suppose that the 
inhabitants of each region underwent a considerable amount of 
modification and extinction, and that there was much migration 
from other parts of the world. As we have reason to believe that 
large areas are affected by the same movement, it is probable that 
strictly contemporaneous formations have often been accumulated 
over very wide spaces in the same quarter of the world ; but we are 
very far from having any right to conclude that this has invariably 
been the case, and that large areas have invariably been affected by 
the same movements. When two formations have been deposited 
in two regions during nearly, but not exactly, the same period, we 
should find in both, from the causes explained in the foregoing 
paragraphs, the same general succession in the forms of life ; but 
the species would not exactly correspond ; fcr there will have been 
a little more time in the one region than in the other for modifica- 
tion, extinction, and immigration. 

I suspect that cases of this nature occur in Europe. Mr. 
Prestwich, in his admirable Memoirs on the eocene deposits of 
England and France, is able to draw a close general parallelism 
between the successive stages in the two countries; but when be 


compares certain stages in England with those in France, although 
he finds in both a curious accordance in the numbers of the species 
belonging to the same genera, yet the species themselves differ in 
a manner very difficult to account for considering the proximity 
of the two areas, — unless, indeed, it be assumed that an isthmus 
separated two seas inhabited by distinct, but contemporaneous, 
faunas. Lyell has made similar observations on some of the later 
tertiary formations. Barrande, also, shows that there is a striking 
general parellelism in the successive Silurian deposits of Bohemia 
and Scandinavia ; nevertheless he finds a surprising amount of 
difference in the species. If the several formations in these regions 
have not been deposited during the same exact periods, — a forma- 
tion in one region often corresponding with a blank interval in the 
other, — and if in both regions the species have gone on slowly 
changing during the accumulation of the several formations and 
during the long intervals of time between them ; in this case the 
several formations in the two regions could be arranged in the same 
order, in accordance with the general succession of the forms of life, 
and the order would falsely appear to be strictly parallel ; never- 
theless the species would not be all the same in the apparently 
corresponding stages in the two regions. 

On the AJJinities of Extinct Species to each other, and to Living 


Let us now look to the mutual affinities of extinct and living 
species. All fall into a few grand classes ; and this fact is at once 
explained on the principle of descent. The more ancient any form 
is, the more, as a general rule, it differs from living forms. But, 
as Buckland long ago remarked, extinct species can all be classed 
either in still existing groups, or between them. That the extinct 
forms of life help to fill up the intervals between existing genera, 
families, and orders, is certainly true ; but as this statement has 
often been ignored or even denied, it may be well to make some 
remarks on this subject, and to give some instances. If we confine 
our attention either to the living or to the extinct species of the 
same class, the series is far less perfect than if we combine both into 
Dne general system. In the writings of Professor Owen we continually 
meet with the expression of generalised forms, as applied to extinct 
animals ; and in the writings of Agassiz, of prophetic or synthetic 
types; and these terms imply that such forms are in fact inter- 
mediate or connecting linl^s. Another distinguished palaeontologist, 
M. Gaudty, has shown in the most striking manner that many of 
the fossil mammals discovered by him in Attica serve to break 


down the intervals between existing genera. Cuvier ranked tiie 
Euminants and Pachyderms, as two of the most distinct orders of 
mammals ; but so many fossil links have been disentombed that 
Owen has had to alter the whole classification, and has placed 
certain pachyderms in the same sub-order with ruminants ; for 
example, he dissolves by gradations the apparently wide interval 
between the pig and the camel. The Ungulata or hoofed quad- 
rupeds are now divided into the even-toed or odd-toed divisions; 
but the Macrauchenia of S. America connects to a certain extent 
these two grand divisions. No one will deny that the Hipparion is 
intermediate between the existing horse and certain older ungulate 
forms. What a wonderful connecting link in the chain of mammals 
is the Typotherium from S. America, as the name given to it by 
Professor Gervais expresses, and which cannot be placed in any 
existing order. The Sirenia form a very distinct group of mammals, 
and one of the most remarkable peculiarities in the existing dugong 
and lamentin is the entire absence of hind limbs, without even a 
rudiment being left ; but the extinct Halitherium had, according 
to Professor Flower, an ossified thigh-bone " articulated to a well- 
defined acetabulum in the pelvis," and it thus makes some approach 
to ordinary hoofed quadrupeds, to which the Sirenia are in other 
respects allied. The cetaceans or whales are widely different from 
all other mammals, but the tertiary Zeuglodon and Squalodon, 
which have been placed by some naturalists in an order by them- 
selves, are considered by Professor Huxley to be undoubtedly ceta- 
ceans, " and to constitute connecting links with the aquatic car- 

Even the wide interval between birds and reptiles has been 
shown by the naturalist just quoted to be partially bridged over in 
the most unexpected manner, on the one hand, by the ostrich and 
extinct Archeopteryx, and on the other hand, by the Compso- 
gnathus, one of the Dinosaurians — that group which includes the 
most gigantic of all terrestrial reptiles. Turning to the Inverte- 
brata, Barrande asserts, and a higher authority could not be named, 
that he is every day taught that, although paleozoic animals can 
certainly be classed under existing groups, yet that at this ancient 
period the groups were not so distinctly separated from each other 
as they now are. 

Some writers have objected to any extinct species, or group of 
species, being considered as intermediate between any two living 
species, or groups of species. If by this term it is meant that 
an extinct form is directly intermediate in all its characters be- 
tween two living forms or groups, the objection is probably valid 


But in a natural classification many fossil species certainly stand 
between living species, and some extinct genera between living 
genera, even between genera belonging to distinct families. The 
most common case, especially with respect to very distinct groups, 
such as fish and reptiles, seems to be, that, supposing them to be 
distinguished at the present day by a score of characters, the ancient 
members are separated by a somewhat lesser number of characters ; 
so that the two groups formerly made a somewhat nearer approach 
to each other than they now do. 

It is a common belief that the more ancient a form is, by so 
much the more it tends to connect by some of its characters groups 
now widely separated from each other. This remark no doubt 
must be restricted to those groups which have undergone much 
change in the course of geological ages ; and it would be difficult 
to prove the truth of the proposition, for every now and then even 
a living animal, as the Lepidosiren, is discovered having affinities 
directed towards very distinct groups. Yet if we compare the 
older Eeptiles and Batrachians, the older Fish, the older Cepha- 
lopods, and the eocene Mammals, with the more recent members 
of the same classes, we must admit that there is truth in the 

Let us see how far these several facts and inferences accord with 
the theory of descent with modification. As the subject is some- 
what complex, I must request the reader to turn to the diagram 
in the fourth chapter. We may suppose that the numbered letters 
in italics represent genera, and the dotted lines diverging from 
ihem the species in each genus. The diagram is much too simple, 
too few genera and too few species being given, but this is un- 
important for us. The horizontal lines may represent successive 
geological formations, and all the forms beneath the uppermost 
line may be considered as extinct. The three existing genera 
a", <2^*, j:)^^ will form a small family; ?>" and/" a closely allied 
family or sub-family ; and o^*, e", m^^, a third family. These 
three families, together with the many extinct genera on the 
several lines of descent diverging from the parent-form (A) will 
form an order ; for all will have inherited something in common 
from their ancient progenitor. On the principle of the continued 
tendency to divergence of character, which was formerly illus- 
trated by this diagram, the more recent any form is, the more 
it will generally difier from its ancient progenitor. Hence we 
can understand the rule that the most ancient fossils differ most 
from existing forms. We must not, however, assume that diver- 
gence of character is a necessary contingency ; it depends solely 


on the descendants from a species being thus enabled to seize on 
many and different places in the economy of nature. Therefore it 
is quite possible, as we have seen in the case of some Silurian forms, 
that a species might go on being slightly modified in relation to 
its slightly altered conditions of life, and yet retain throughout a 
vast period the same general characteristics. This is represented 
in the diagram by the letter f". 

All the many forms, extinct and recent, descended frcm (A), 
make, as before remarked, one order; and this order, from the 
continued effects of extinction and divergence of character, has 
become divided into several sub-families and families, some of 
which are supposed to have perished at different periods, and some 
to have endured to the present day. 

By looking at the diagram we can see that if many cf the extinct 
forms supposed to be imbedded in the successive formations, were 
discovered at several points low down in the series, the three 
existing families on the uppermost line would be rendered less 
distinct from each other. If, for instance, the genera a^, a^, a^^, 
/■*, m^, m^, m^, were disinterred, these three families would be so 
closely linked together that they probably would have to be united 
into one great family, in nearly the same manner as has occurred 
with ruminants and certain pachyderms. Yet he who objected to 
consider as intermediate the extinct genera, which thus link together 
the living genera of three families, would be partly justified, for 
they are intermediate, not directly, but only by a long and cir- 
cuitous course through many widely different forms. If many 
extinct forms were to be discovered above one of the middle 
horizontal lines or geological formations — for instance, above 
No. VI. — but none from beneath this line, then only two of 
the families (those on the left hand, a^*, &c., and h^*, &c.) would 
have to be united into one ; and there would remain two families, 
which would be less distinct from each other than they were 
before the discovery of the fossils. So agam if the three families 
formed of eight genera (a^* to m^*), on the uppermost line, be 
supposed to differ from each other by half-a-dozen important 
characters, then the families which existed at the period marked 
YI. would certainly have differed from each other by a less number 
of characters ; for they would at this early stage of descent have 
diverged in a less degree from their common progenitor. Thus it 
comes that ancient and extinct genera are often in a greater or less 
degree intermediate in character between their modified descendants, 
or between their collateral relations. 

Under nature the process will be far more complicated than is 


represented in the diagram; for the groups will have been raore 
numerous ; they will have endured for extremely unequal lengths 
of time, and will have been modified in various degrees. As we 
possess only the last volume of the geological record, and that in 
a very broken condition, we have no right to expect, except 
in rare cases, to fill up the wide intervals in the natural system, 
and thus to unite distinct families or orders. All that we have 
a right to expect is, that those groups which have, within known 
geological periods, undergone much modification, should in the 
older formations make some slight approach to each other ; so 
that the older members should differ less from each ' other in 
some of their characters than do the existing members of the 
same groups; and this by the concurrent evidence of our best 
paheontologists is frequently the case. 

Thus, on the theory of descent with modification, the main facts 
with respect to the mutual affinities of the extinct forms of life 
to each other and to living forms, are explained in a satisfactory 
manner. And they are wholly inexplicable on any other view. 

On this same theory, it is evident that the fauna during any 
one great period in the earth's history will be intermediate in 
general character between that which preceded and that which 
succeeded it. Thus the species which lived at the sixth great 
stage of descent in the diagram are the modified offspring of those 
which lived at the fifth stage, and are the parents of those which 
"became still more modified at the seventh stage ; hence they could 
hardly fail to be nearly intermediate in character between the forms 
of life above and below. We must, however, allow for the entire 
extinction of some preceding forms, and in any one region for the 
immigration of new forms from other regions, and for a large 
amount of modification during the long and blank intervals between 
the successive formations. Subject to these allowances, the fauna 
of each geological period undoubtedly is intermediate in character*, 
between the preceding and succeeding faunas. I need give only 
one instance, namely, the manner in which the fossils of the 
Devonian system, when this system was first discovered, were at 
once recognised by palaeontologists as intermediate in character 
between those of the overlying carboniferous, and underlying 
Silurian systems. But each fauna is not necessarily exactly inter- 
mediate, as unequal intervals of time have elapsed between con*- 
sccutive formations. 

It is no real objection to the truth of the statement that the 
fauna of each period as a whole is nearly intermediate in character 
between the preceding and succeeding faunas, that certain genera 


ofifer exceptions to the rule. For instance, the species of mastodons 
and elephants, when arranged by Dr. Falconer in two series, — in 
the first place according to their mutual affinities, and in the second 
place according to their periods of existence, — do not accord in 
arrangement. The species extreme in character are not the oldest 
or the most recent ; nor are those which are intermediate in cha- 
racter, intermediate in age. But supposing for an instant, in this 
and other such cases, that the record of the first appearance and 
disappearance of the species was complete, which is far from the 
case, we have no reason to believe that forms successively produced 
necessarily endure for coiTesponding lengths of time. A very 
ancient form may occasionally have lasted much longer than a form 
elsewhere subsequently produced, especially in the case of terres- 
trial productions inhabiting separated districts. To compare small 
things with great ; if the principal living and extinct races of the 
domestic pigeon were arranged in serial affinity, this arrangement 
would not closely accord with the order in time of their production, 
and even less with the order of their disappearance ; for the parent 
rock-pigeon still lives ; and many varieties between the rock-pigeon 
and the carrier have become extinct; and carriers which are 
extreme in the important character of length of beak originated 
earlier than short-beaked tumblers, which are at the opposite end 
of the series in this respect. 

Closely connected with the statement, that the organic remains 
from an intermediate formation are in some degree intermediate 
in character, is the fact, insisted on by all palaeontologists, that 
fossils from two consecutive formations are far more closely related 
to each other, than are the fossils from two remote formations. 
Pictet gives as a well-known instance, the general resemblance of 
the organic remains from the several stages of the Chalk forma- 
tion, though the species are distinct in each stage. This fact alone, 
from its generality, seems to have shaken Professor Pictet in 
his belief in the immutability of species. He who is acquainted 
with the distribution of existing species over the globe, will not 
attempt to account for the close resemblance of distinct species in 
closely consecutive formations, by the physical conditions of the 
ancient areas having remained nearly the same. Let it be remem- 
bered that the forms of life, at least those inhabiting the sea, have 
changed almost simultaneously throughout the world, and there- 
fore under the most different climates and conditions. Consider 
the prodigious vicissitudes of climate during the pleistocene period, 
which includes the whole glacial epoch, and note how little the 
epecific forms of the inhabitants of the sea have been affected. 


On the theory of descent, the full meaning of the fossil remains 
from closely consecutive formations being closely related, though 
ranked as distinct species, is obvious. As the accumulation of each 
formation has often been interrupted, and as long blank intervals 
have intervened between successive formations, we ought not to 
expect to find, as I attempted to show in the last chapter, in any 
one or in any two formations, all the intermediate varieties between 
the species which appeared at the commencement and close of these 
periods : but we ought to find after intervals, very long as measured 
by years, but only moderately long as measured geologically, 
closely allied forms, or, as they have been called by some authors, 
representative species ; and these assuredly we do find. We find, 
in short, such evidence of the slow and scarcely sensible mutation? 
of specific forms, as we have the right to expect. 

On the State of Develojpment of Ancient compared with Living 


We have seen in the fourth chapter that the degree of differentia- 
tion and specialisation of the parts in organic beings, when arrived 
at maturity, is the best standard, as yet suggested, of their degree 
of perfection or highness. We have also seen that, as the speciali- 
sation of parts is an advantage to each being, so natural selection 
will tend to render the organisation of each being more specialised 
and perfect, and in this sense higher ; not but that it may leave 
many creatures with simple and unimproved structures fitted for 
simple conditions of life, and in some cases will even degrade or 
simplify the organisation, yet leaving such degraded beings better 
fitted for their new walks of life. In another and more general 
manner, new species become superior to their predecessors ; for they 
have to beat in the struggle for life all the older forms, with which 
they come into close competition. We may therefore' conclude 
that if under a nearly similar climate the eocene inhabitants of the 
world could be put into competition with the existing inhabitants, 
the former would be beaten and exterminated by the latter, as 
would the secondary by the eocene, and the pala3ozoic by the 
secondary forms. So that by this fundamental test of victory in 
the battle for life, as well as by the standard of the specialisation of 
organs, modern forms ought, on the theory of natural selection, to 
stand higher than ancient forms. Is this the case? A large 
majority of palaeontologists would answer in the affirmative ; and it 
seems that this answer must be admitted as true, though difiicult of 

It is no valid objection ' o this conclusion, that certain Brachiopcds 

X 2 


have been but slightly modified from an extremely remote geological 
epoch ; and that certain land and fresh-water shells have remained 
nearly the same, from the time when, as far as is known, they first 
appeared. It is not an insuperable difiiculty that Foraminifera have 
not, as insisted on by Dr. Carpenter, progressed in organisation since 
even the Laurentian epoch ; for some organisms would have to 
remain fitted for simple conditions of life, and what could be better 
fitted for this end than these lowly organised Protozoa ? Such 
objections as the above would be fatal to my view, if it included 
advance in organisation as a necessary contingent. They would 
likewise be fatal, if the above Foraminifera, for instance, could be 
proved to have first come into existence during the Laurentian 
epoch, or the above Brachiopods during the Cambrian formation ; for 
in this case, there would not have been time sufficient for thu 
development of these organisms up to the standard which they 
had then reached. When advanced up to any given point, there is 
no necessity, on the theory of natural selection, for their further 
continued progress ; though they will, during each successive age, 
have to be slightly modified, so as to hold their places in relation to 
slight changes in their conditions. The foregoing objections hinge 
on the question whether we really know how old the world is, and 
at what period the various forms of life first appeared; and this 
may well be disputed. 

The problem whether organisation on the whole has advanced is 
in many ways excessively intricate. The geological record, at all 
times imperfect, does not extend far enough back, to show with 
unmistakeable clearness that within the known history of the world 
organisation has largely advanced Even at the present day, looking 
to members of the same class, naturalists are not unanimous which 
forms ought to be ranked as highest : thus, some look at the 
selaceans or sharks, from their approach in some important points 
of structure to reptiles, as the highest fish ; others look at the 
teleosteans as the highest. The ganoids stand intermediate between 
the selaceans and teleosteans ; the latter at the present day are 
largely preponderant in number ; but formerly selaceans and 
ganoids alone existed ; and in this case, according to the standard 
of highness chosen, so will it be said that fishes have advanced cr 
retrograded in organisation. To attempt to compare members ci 
distinct types in the scale of highness seems hopeless ; who will 
decide whether a cuttle-fish be higher than a bee — that insect which 
the great Von Baer believed to be " in fact more highly organised 
than a fish, although upon another type " ? In the complex struggle 
for life it is quite credible that crustaceans, not very high in their 


own class, might beat ceplialopods, the highest molluscs ; and such 
crustaceans, though not highly developed, would stand very high in 
the scale of invertebrate animals, if judged by the most decisive of 
all trials — the law of battle. Besides these inherent difficulties in de- 
ciding which forms are the most advanced in organisation, we ought 
not solely to compare the highest members of a class at any two 
periods — though undoubtedly this is one and perhaps the most 
important element in striking a balance — but we ought to compare 
all the members, high and low, at the two periodis. At an ancient 
epoch the highest and lowest molluscoidal animals, namely, cephalo- 
pods and brachiopods, swarmed in numbers ; at the present time both 
groups are greatly reduced, whilst others, intermediate in organisatioUj 
have largely increased; consequently some naturalists maintain 
that molluscs were formerly more highly developed than at present ; 
but a stronger case can be made out on the opposite side, by con- 
sidering the vast reduction of brachiopods, and the fact that our 
existing cephalopods, though few in number, are more highly orga- 
nised than their ancient representatives. We ought also to compare 
the relative proportional numbers at any two periods of the high and 
low classes throughout the world : if, for instance, at the present 
day fifty thousand kinds of vertebrate animals exist, and if we knew 
that at some former period only ten thousand kinds existed, we 
ought to look at this increase in number in the highest class, which 
implies a great displacement of lower forms, as a decided advancb 
in the organisation of the world. We thus see how hopelessly 
difficult it is to compare with perfect fairness, under such extremely 
complex relations, the standard of organisation of the imperfectly- 
known faunas of successive periods. 

We shall appreciate this difficulty more clearly, by looking to 
certain existing faunas and floras. From the extraordinary manner 
in which European productions have recently spread over New 
Zealand, and have seized on places which must have been prc^viously 
occupied by the indigenes, we must believe, that if all the animals 
and plants of Great Britain were set free in New Zealand, a multi- 
tude of British forms would in the course of time become thoroughly 
naturalised there, and would exterminate many of the natives. On 
the other hand, from the fact that har ily a single inhabitant of the 
southern hemisphere has become wil I in any part of Europe, we 
may well doubt whether, if all the productions of New Zealand were 
set free in Great Britain, any considerable number would be enabled 
to seize on places now occupied by OLr native plants and animals. 
Under this point of view, the productions of Great Britain stand 
much higher in the scale than those of New Zealand. Yet the 


most skillul naturalist, I'rom an examination of the species of the 
two countries, could not have foreseen this result. 

Agassiz and several other highly competent judges insist that 
ancient animals resemble to a certain extent the embryos of recent 
animals belonging to the same classes ; and that the geological suc- 
cession of extinct forms is nearly parallel with the embryological 
development of existing forms. This view accords admirably well 
with our theory. In a future chapter I shall attempt to show that 
the adult differs from its embryo, owing to variations having 
supervened at a not early age, and having been inherited at a 
corresponding age. This process, whilst it leaves the embryo 
almost unaltered, continually adds, in the course of successive 
generations, more and more difterence to the adult. Thus the 
embryo comes to be left as a sort of picture, preserved by nature, of 
the former and less modified condition of the species. This view 
may be true, and yet may never be capable of proof. Seeing, for 
instance, that the oldest known mammals, reptiles, and fishes 
strictly belong to their proper classes, though some of these old 
forms are in a slight degree less distinct from each other than are 
the typical members of the same groups at the present day, it 
would be vain to look for animals having the common embryological 
character of the Yertebrata, until beds rich in fossils are discovered 
far beneath the lowest Cambrian strata — a discovery of which the 
chance is small. 

On the Succession of the same Types within the same Areas, 
during the later Tertiary periods. 

Mr. Clift many years ago showed that the fossil mammals from 
the Australian caves were closely allied to the living marsupials 
of that continent. In South America, a similar relationship is 
manifest, even to an uneducated eye, in the gigantic pieces of 
armour, like those of the armadillo, found in several parts of La 
Plata ; and Professor Owen has shown in the most striking manner 
that most of the fossil mammals, buried there in such numbers, are 
related to South American types. This relationship is even more 
clearly seen in the wonderful collection of fossil bones made by MM. 
Lund and Clausen in the caves of Brazil. I was so much impressed 
with these facts that I strongly insisted, in 1839 and 1845, on this 
*' law of the succession of types," — on " this wonderful relationship 
in the same continent between the dead and the living.'* Professor 
Owen has subsequently extended the same generalisation to the 
mammals of the Old World. We see the same law in this authors 
restorations of the extinct and gigantic birds of New Zealand. We 


see it also in the birds of the caves of Brazil. Mr. Woodward has 
shown that the same law holds good with sea-shells, but, from the 
wide distribution of most molluscs, it is not well displayed by 
them. Other cases could be added, as the relation between the 
extinct and living land-shells of Madeira ; and between the extinct 
and living brackish water-shells of the Aralo-Caspian Sea. 

Now what does this remarkable law of the succession of the 
same types within the same areas mean ? He would be a bold man 
who, after comparing the present climate of Australia and of parts of 
South America, under the same latitude, would attempt to account, 
on the one hand through dissimilar physical conditions, for the 
dissimilarity of the inhabitants of these two continents ; and, on the 
-other hand through similarity of conditions, for the uniformity of 
the same types in each continent during the later tertiary periods. 
Nor can it be pretended that it is an immutable law that marsupials 
should have been chiefly or solely produced in Australia ; or that 
Edentata and other American types should have been solely produced 
in South America. For we know that Europe in ancient times was 
XDCopled by numerous marsupials ; and I have shown in the publi- 
cations above alluded to, that in America the law of distribution of 
terrestrial mammals was formerly different from what it now is. 
North America formerly partook strongly of the present character 
of the southern half of the continent; and the southern half was 
formerly more closely allied, than it is at present, to the northern 
half. In a similar manner we know, from Falconer and Cautley's 
^discoveries, that Northern India was formerly more closely related in 
its mammals to Africa than it is at the present time. Analogous facts 
'Could be given in relation to the distribution of marine animals. 

On the theory of descent with modification, the great law of the 
long enduring, but not immutable, succession of the same types 
within the same areas, is at once explained ; for the inhabitants 
'Of each quarter of the world will obviously tend to leave in that 
•quarter, during the next succeeding period of time, closely allied 
though in some degree modified descendants. If the inhabitants 
^of one continent formerly differed greatly from those of another 
continent, so will their modified descendants still differ in nearly 
the same manner and degree. But after very long intervals of 
time, and after great geographical changes, permitting much inter- 
migration, the feebler will yield to the more dominant forms, and 
there will be nothing immutable in the distribution of organic 

It may be asked in ridicule, whether I suppose that the megathe- 
trium and other aUied huge monsters, which formerly lived in 


South America, have left behind them the sloth, armadillo, and 
anteater, as their degenerate descendants. This cannot for an 
instant be admitted. These huge animals have become wholly 
extinct, and have left no progeny. But in the cavc-s of Brazil, 
there are many extinct species which are closely allied in size and 
in all other characters to the species still living in South America ;, 
and some of these fossils may have been the actual progenitors of 
the living species. It must not be forgotten that, on our theory, 
all the species of the same genus are the descendants of some one 
species ; so that, if six genera, each having eight species, be found in 
one geological formation, and in a succeeding formation there be six 
other allied or representative genera each with the same number of 
species, then we may conclude that generally only one species of each 
of the older genera has left modified descendants, whi-ch constitute 
the new genera containing the several species; the other seven 
species of each old genus having died out and left no progeny. Or, 
and this will be a far commoner case, two or three species in two 
or three alone of the six older genera will be the parents of the new 
genera : the other species and the other whole genera havin«g become 
utterly extinct. In failing orders, with the genera and species 
decreasing in numbers as is the case with the Edentata of South 
America, still fewer genera and species will leave modified blood- 

Summary of the preceding and present Chapters. 

I have attempted to show that the geological record is extremely 
imperfect ; that only a small portion of the globe has been geo- 
logically explored with care ; that only certain classes of organic 
beings have been largely preserved in a fossil state ; that the 
number both of specimens and of species, preserved in our museums, 
is absolutely as nothing compared with the number of generations 
which must have passed away even during a single formation ; that, 
owing to subsidence being almost necessary for the accumulation 
of deposits rich in fossil species of many kinds, and thick enough to 
outlast future degradation, great intervals of time must have elapsed 
between most of our successive formations ; that there has probably 
been more extinction during the periods of subsidence, and more 
variation during the periods of elevation, and during the latter tho 
record will have been least perfectly kept; that each single forma- 
tion has not been continuously deposited ; that the duration of 
each formation is, probably short compared with the average dura- 
tion of specific forms ; that migration has played an important part 
in the first appearance of new forms in any one area and formation • 


that widely ranging species are those which have varied mcst fre- 
quently, and have oftenest given rise to new species ; that varieties 
have at first been local; and lastly, although each species must 
have passed through numerous transitional stages, it is probable 
that the periods, during which each underwent modification, though 
many and long as measured by years, have been short in com- 
parison with the periods during which each remained in an un- 
changed condition. These causes, taken conjointly, will to a large 
extent explain why — though we do find many links — we do not 
find interminable varieties, connecting together all extinct and 
existing forms by the finest graduated steps. It should also be 
constantly borne in mind that any linking variety between two 
forms, which might be found, would be ranked, unless the whole 
chain could be perfectly restored, as a new and distinct species ; 
for it is not pretended that we have any sure criterion by which 
species and varieties can be discriminated. 

He who rejects this view of the imperfection of the geological 
record, will rightly reject the whole theory. For he may ask in 
vain where are the numberless transitional links which must 
formerly have connected the closely allied or representative species, 
found in the successive stages of the same great formation ? He 
may disbelieve in the immense intervals of time which must have 
elapsed between our consecutive formations ; he may overlook how 
important a part migration has played, when the formations of any 
one great region, as those of Europe, are considered ; he may urge 
the apparent, but often falsely apparent, sudden coming in of whole 
groups of species. He may ask where are the remains of those 
infinitely numerous organisms which must have existed long before 
the Cambrian system was deposited ? We now know that at least 
one animal did then exist; but I can answer this last question 
only by supposing that where our oceans now extend they have 
extended for an enormous period, and where our oscillating con- 
tinents now stand they have stood since the commencement of the 
Cambrian system ; but that, long before that epoch, the world pre- 
sented a widely different aspect ; and that the older continents, 
formed of formations older than any known to us, exist now only 
as remnants in a metamorphosed condition, or lie still buried under 
the ocean. 

Passing from these difficulties, the other great leading facts in 
palajontology agree admirably with the theory of descent with 
modification through variation and natural selection. We can thus 
understand how it is that new species come in slowly and succes- 
sively ; how species of different classes do not necessarily change 

ol4 SU]\niAKY OF THK [Chap. XL 

together, or at the same rate, or in the same degree ; yet in the 
long run that all undergo modification to some extent. The ex- 
tinction of old forms is the almost inevitable consequence of the 
production of new forms. We can understand why when a species 
has once disappeared it never reappears. Groups of species increase 
in numbers slowly, and endure for unequal periods of time ; for tiie 
process of modification is necessarily slow, and depends on many 
complex contingencies. The dominant species belonging to large 
and dominant groups tend to leave many modified descendants, 
which form new sub-groups and groups. As these are formed, the 
species of the less vigorous groups, from their inferiority inherited 
from a common progenitor, tend to become extinct together, and 
to leave no modified offspring on the face of the earth. But the 
utter extinction of a whole group of species has sometimes been 
a slow process, from the survival of a few descendants, lingering 
in protected and isolated situations. When a group has once wholly 
disappeared, it do^s not reappear ; for the link of generation has 
been broken. 

We can understand how it is that dominant forms which spread 
widely and yield the greatest number of varieties tend to people the 
world with allied, but modified, descendants ; and these will gene- 
rally succeed ^n displacing the groups which are their inferiors in 
the struggle for existence. Hence, after long intervals of time, the 
productions of the world appear to have changed simultaneously. 

We can understand how it is that all the forms of life, ancient 
and recent, make together a few grand classes. We can under- 
stand, from the continued tendency to divergence of character,. why 
the more ancient a form is, the more it generally differs from those 
now living ; why ancient and extinct forms often tend to fill up 
gaps between existing forms, sometimes blending two groups, jore- 
viously classed as distinct, into one ; but more commonly bringing 
them only a little closer together. The more ancient a form is, the 
more often it stands in some degree intermediate between groups 
now distinct ; for the more .ancient a form is, the more nearly it 
will be related to, and consequently resemble, the common pro- 
genitor of groups, since become widely divergent. Extinct forms 
are seldom directly intermediate between existing forms ; but are 
intermediate only by a long and circuitous course through other 
extinct and different forms. We can clearly see why the organic 
remains of closely consecutive formations are closely allied ; for 
they are closely linked together by generation. We can clearly see 
why the remains of an intermediate formation are intermediate in 


The inhabitants of the world at each successive period in its 
history have beaten their predecessors in the race for life, and 
are, in so far, higher in the scale, and their structure has gene- 
rally become more specialised ; and this may account for the com- 
mon belief held by so many palaeontologists, that organisation on 
the whole has progressed. Extinct and ancient animals resemble 
to a certain extent the embryos of the more recent animals belong- 
ing to the same classes, and this wonderful fact receives a simple 
■explanation according to our views. The succession of the same 
types of structure within the same areas during the later geological 
periods ceases to be mysterious, and is intelligible on the principle 
of inheritance. 

If then the geological record be as imperfect as many believe, and 
it may at least be asserted that the record cannot be proved to be 
much more perfect, the main objections to the theory of natural 
selection are greatly diminished or disappear. On the other hand, 
all the chief laws of palseontology plainly proclaim, as it seems to 
ine, that species have been produced by ordinary generation : old 
forms having been supplanted by new and improved forms of life, 
the products of Yariation and the Survival of the Tittost. 




Geographical Distribution. 

Present distribution cannot be accounted for by ditierences in physica. 
conditions — Importance of barriers — Affinity of the productions of the 
same continent — Centres of creation — Means of dispersal by changes 
of climate and of the level of the land, and by occasional means — 
Dispersal during the Glacial period — Alternate Glacial periods in the 
North and South. 

In considering the distribution of organic beings over the face of 
the globe, the first great fact which strikes lis is, that neither the 
similarity nor the dissimilarity of the inhabitants of various regions 
can be wholly accounted for by climatal and other physical con- 
ditions. Of late, almost every author who has studied the subject 
has come to this conclusion. The case of America alone would 
almost suffice to prove its truth : for if we exclude the arctic 
and northern temperate parts, all authors agree that one of the 
most fundamental divisions in geographical distribution is that 
between the New and Old Worlds; yet if we travel over the 
vast American continent, from the central parts of the United 
States to its extreme southern point, we meet with the most 
diversified conditions; humid districts, arid deserts, lofty moun- 
tains, grassy plains, forests, marshes, lakes, and great rivers, under 
almost every temperature. There is hardly a climat© or condition 
in the Old World which cannot be paralleled in the New — at 
least as closely as the same species generally require. No doubt 
small areas can be pointed out in the Old World hotter than any in 
the New World, but these are not inhabited by a fauna different 
from that of the surrounding districts ; for it is rare to find a group 
of organisms confined to a small area, of which the conditions are 
peculiar in only a slight degree. Notwithstanding this general 
parallelism in the conditions of the Old and New Worlds, how 
widely different are their living productions ! 

In the southern hemisphere, if we compare large tracts of land m 
Australia, South Africa, and western South America, between lati- 
tudes 25° and 35°, we shall find parts extremely similar in all their 


conditions, yet it would not be possible to point out three faunas 
and floras more utterly dissimilar. Or, again, we may compare the 
productions of South America south of lat. 35° with those north of 
25°, which consequently are separated by a space of ten degrees 
of latitude and are exposed to considerably different conditions, yet 
they are incomparably more closely related to each other than they 
are to the productions of Australia or Africa under nearly the same 
climate. Analogous facts could be given with respect to the inha- 
bitants of the sea. 

A second great fact which strikes us in our general review is, 
that barriers of any kind, or obstacles to free migration, are related 
in a close and important manner to the differences between the 
productions of various regions. We see this in the great difference 
in nearly all the terrestrial productions of the New and Old Worlds, 
excepting in the northern parts, where the land almost joins, and 
where, under a slightly different climate, there might have been 
free migration for the northern temperate forms, as there now is for 
the strictly arctic productions. We see the same fact in the great 
difference between the inhabitants of Australia, Africa, and South 
America under the same latitude ; for these countries are ahnost as 
much isolated from each other as is possible. On each continent, 
also, we see the same fact ; for on the opposite sides of lofty and 
continuous mountain-ranges, of great deserts, and even of large 
rivers, we find different productions ; though as mountain-chains, 
deserts, &c., are not as impassable, or likely to have endured so 
long, as the oceans separating continents, the differences are very 
inferior in degree to those characteristic of distinct continents. 

Turning to the sea, we find the same law. The marine inha- 
bitants of the eastern and western shores of South America are 
very distinct, with extremely few shells, Crustacea or echinoder- 
mata in common ; but Dr. Glinthcr has recently shown that about 
thirty per cent, of the fishes are the same on the opposite sides 
of the isthmus of Panama; and this fact has led naturalists to 
believe that the isthmus was formerly open. Westward of the 
shores of America, a wide space of open ocean extends, with not 
an island as a halting-place for emigrants ; here we have a barrier 
of another kind, and as soon as this is passed we meet in the eastern 
islands of the Pacific with another and totally distinct fauna. So 
that three marine faunas range far northward and southward in 
parallel lines not far from each other, under corresponding climates ; 
but from being separated from each other by impassable barriers 
either of land or open sea, they are almost wholly distinct. On the 
other hand, proceeding still farther westward from the eastern 


islands of the tropical parts of the Pacific, we encounter no im- 
passable barriers, and we have innumerable islands as halting- 
]3laces, or continuous coasts, until, after travelling over a hemisphere, 
we come to the shores of Africa ; and over this vast space we meet 
with no well-defined and distinct marine faunas. Although so few 
marine animals are common to the above-named three approximate 
faunas of Eastern and Western America and the eastern Pacific 
islands, yet many fishes range from the Pacific into the Indian 
Ocean, and many shells are common to the eastern islands of the 
Pacific and the eastern shores of Africa on almost exactly opposite 
meridians of longitude. 

A third great fact, partly included in the foregoing statement, is 
the affinity of the productions of the same continent or of the same 
sea, though the species themselves are distinct at different points 
and stations. It is a law of the widest generality, and every con- 
tinent offers innumerable instances. Nevertheless the naturalist, in 
travelling, for instance, from north to south, never fails to be struck 
by the manner in which successive groups of beings, specifically 
distinct, though nearly related, replace each other. He hears from 
closely allied, yet distinct kinds of birds, notes nearly similar, and 
sees their nests similarly constructed, but not quite alike, with eggs 
coloured in nearly the same manner. The plains near the Straits of 
Magellan are inhabited by one species of Khea (American ostrich), 
and northward the plains of La Plata by another species of the same 
genus ; and not by a true ostrich or emu, like those inhabiting 
Africa and Australia under the same latitude. On these same plains 
of La Plata, we see the agouti and bizcacha, animals having nearly 
the same habits as our hares and rabbits and belonging to the same 
order of Rodents, but they plainly display an American type of 
structure. We ascend the lofty peaks of the Cordillera, and we find 
an alpine species of bizcacha ; we look to the waters, and we do not 
find the beaver or musk-rat, but the coypu and capybara, rodents 
of the S. American type. Innumerable other instances could be 
given. If we look to the islands off the American shore, however 
much they may differ in geological structure, the inhabitants are 
essentially American, though they may be all peculiar species. We 
may look back to past ages, as shown in the last chapter, and we 
find American types then prevailing on the American continent and 
in the American seas. We see in these facts some deep organic 
bond, throughout space and time, over the same areas of land and 
water, independently of physical conditions. The naturalist must 
be dull, who is not led to inquire what this bond is. 

The bond is simply inheritance, that cause which alone, as far as 


we positively know, produces organisms quite like each other, or,, 
as we see in the case of varieties, nearly alike. The dissimilarity of 
the inhabitants of different regions may be attributed to modification 
through variation and natural selection, and probably in a suV 
ordinate degree to the definite infiuence of different physical con- 
ditions. The degrees of dissimilarity will depend on the migration 
of the more dominant forms of life from one region into another 
having been more or less effectually prevented, at periods more or 
less remote ; — on the nature and number of the former immigrants ; 
— and on the action of the inhabitants on each other in leading to 
the preservation of different modifications ; the vplation of organism 
to organism in the struggle for life being, as I have already often 
remarked, the most important of all relations. Thus the high im- 
portance of barriers comes into play by checking migration ; as does 
time for the slow process of modification through natural selection.- 
Widely-ranging species, abounding in individuals, which have already 
triumphed over many competitors in their own widely-extended 
homes, will have the best chance of seizing on new places, when they 
spread into new countries. In their new homes they will be ex- 
posed to new conditions, and will frequently undergo further modi- 
fication and improvement ; and thus they will become still further 
victorious, and will produce groups of modified descendants. On this 
principle of inheritance with modification, we can understand how it 
is that sections of genera, whole genera, and even families, are con- 
fined to the same areas, as is so commonly and notoriously the case. 

There is no evidence, as was remarked in the last chapter, of the 
existence of any- law of necessary development. As the variability 
of each species is an independent property, and will be taken advan- 
tage of by natm-aL selection, only so far as it profits each individual 
in its complex struggle for life, so the amount of modification in 
different species will be no uniform quantity. If a number of species, 
after having long competed with each other in their old home, were 
to migrate in a body into a new and afterwards isolated country, 
they would be little liable to modification ; for neither migration 
nor isolation in themselves efi'ect anything. These principles come 
into play only by bringing organisms into new relations with each 
other, and in a lesser degree with the surrounding physical conditions. 
As we have seen in the last chapter that some forms have retained 
nearly the same character from an enormously remote geological 
period, so certain species have migrated over vast spaces, and have 
not become greatly or at all modified. 

According to these views, it is obvious that the several species of 
the same genuS; though inhabiting the most distant quarters of the 


world, must originally have proceeded from the same source, as they 
are descended from the same progenitor. In the case of those 
sjDCcies, which have undergone during v^^hole geological periods httle 
;3iodificaticQ, there is not much difficulty in believing that they have 
migrated from the same region ; for during the vast geographical 
and climatal changes v^hich have supervened since ancient times, 
almost any amount of migration is possible. But in many other 
cases, in which we have reason to believe that the species of a genus 
have been produced within comparatively recent times, there ia 
great difficulty on this head. It is also obvious that the individuals 
of the same species, though now inhabiting distant and isolated 
regions, must have proceeded from one spot, where their parents 
were first produced : for, as has been explained, it is incredible that 
individuals identically the same should have been produced from 
parents specifically distinct. 

Single Centres of supposed Creation. — We are thus brought to 
the question which has been largely discussed by naturalists, namely, 
whether species have been created at one or more points of the 
earth's surface. Undoubtedly there are many cases of extreme 
difficulty in understanding how the same species could possibly 
liave migrated from some one point to the several distant and isolated 
points, where now found. Nevertheless the simplicity of the view 
that each species was first produced within a single region captivates 
the mind. He who rejects it, rejects the vera causa of ordinary 
generation with subsequent migration, and calls in the agency of a 
miracle. It is universally admitted, that in most cases the area 
inhabited by a species is continuous; and that when a plant or 
animal inhabits two points so distant from each other, or with an 
interval of such a nature, that the space could not have been easily 
passed over by migration, the fact is given as something remarkable 
and exceptional. The incapacity of migrating across a wide sea is 
more clear in the case of terrestrial mammals than perhaps with any 
other organic beings; and, accordingly, we find no inexplicable 
instances of the same mammals inhabiting distant points of the 
world. No geologist feels any difficulty in Great Britain possessing 
the same quadrupeds with the rest of Europe, for they were no 
doubt once united. But if the same species can be produced at two 
separate points, why do we not find a single mammal common tt 
Europe and Australia or South America ? The conditions of life 
are nearly the same, so that a multitude of European animals and 
plants have become naturalised in America and Australia ; and some 
of the aboriginal plants are identically the same at these distant 
points of the northern and southern hemispheres ? The answer, as 


I believe, is, that mammals have not been able to migrate, nhereas 
some plants, from their varied means of dispersal, have migrated 
across ihe wide and broken interspaces. The great and striking 
influence of barriers of all kinds, is intelligible only on the view 
that the great majority of species have been produced on one side, 
and have not been able to migrate to the opposite side. Some few 
families, many sub-families, very many genera, and a still greater 
number of sections of genera, are confined to a single region ; and 
it has been observed by several naturalists, that the most natural 
gen(;ra, or those genera in which the species are most closely related 
to each other, are generally confined to the same country, or if they 
have a wide range that their range is continuous. What a strange 
anomaly it would be, if a directly opposite rule were to prevail, 
when we go down one step lower in the series, namely, to the 
individuals of the same species, and these had not been, at least at 
first, confined to some one region ! 

Hence it seems to me, as it has to many otber naturalists, that 
the view of each species having been produced in one area alone, 
and having subsequently migrated from that area as far as its 
powers of migration and subsistence under past and present con- 
ditions permitted, is the most probable. Undoubtedly many cases 
occur, in which we cannot explain how the same species could have 
passed from one point to the other. But the geographical and 
climatal changes, which have certainly occurred within recent 
geological times, must have rendered discontinuous the formerly 
continuous range of many species. So that we are reduced to con- 
sider whether the exceptions to continuity of range are so numerous 
and of so grave a nature, that we ought to give up the belief, 
rendered probable by general considerations, that each species has 
been produced within one area, and has migrated thence as far as it 
could. It would be hopelessly tedious to discuss all the exceptional 
cases of the same species, now living at distant and separated points, 
nor do I for a moment pretend that any explanation could be offered 
of many instances. But, after some preliminary remarks, I will 
discuss a few of the most striking classes of facts ; namely, the 
existence of the same species on the summits of distant mountain- 
ranges, and at distant points in the arctic and antarctic regions ; and 
secondly (in the following chapter), the wide distribution of fresh- 
water productions ; and thirdl}^ the occurrence of the same terrestrial 
species on islands ^nd on the nearest mainland, though separated by 
hundreds of miles of open sea. If the existence of the same species 
at distant and isolated points of the earth's surface, can in many 
instances be explained on the view of each species having migrated 



from a siugle birthplace ; then, considering our ignorance with re- 
Bpect to former climatal and geographical changes and to the various 
fxxjasional m?ans of transport, the hehef that a single birthplace is 
the law, seems to me incomparably the safest. 

In discussing this subject, we shall be enabled at the same time 
to consider a point equally important for us, namely, whether the 
several species of a genus, which must on our theory all be descended 
from a common progenitor, can have migrated, undergoing modi- 
fication during their migration, from some one area. If, when most 
of the species inhabiting one region are different from those of 
p.cother region, though closely allied to them, it can be shown that 
migration from the one region to the other has probably occurred 
at some former period, our general view will be much strengthened ; 
for the explanation is obvious on the principle of descent with modi- 
fication. A volcanic island, for instance, upheaved and formed at 
the distance of a few hundreds of miles from a continent, would 
probably receive from it in the course of time a few colonists, 
and their descendants, though modified, would still be related by 
inheritance to the inhabitants of -that continent. Cases of this 
nature are common, and are, as we shall hereafter see, inexplicable 
on the theory of independent creation. This view of the relation of 
the species of one region to those of another, does not differ much 
from that advanced by Mr. Wallace, who concludes that " every 
species has come into existence coincident both in space and time 
with a pre-existing closely allied species." And it is now well 
known that he attributes this coincidence to descent with modi- 

The question of single or multiple centres of creation differs 
from another though allied question, — namely, whether all the 
individuals of the same species are descended from a single pair, or 
single hermaphrodite, or whether, as some authors suppose, from 
many individuals simultaneously created. With organic beings 
which never intercross, if such exist, each species must be de- 
scended from a succession of modified varieties, that have sup- 
planted each other, but have never blended with other individuals 
or varieties of the same species ; so that, at each successive stage 
of modification, all the individuals of the same form will be de- 
scended from a single parent. But in the great majority of cases, 
namely, with all organisms which habitually unite for each birth, 
or which occasionally intercross, the individuals of the same species 
inhabiting the same area will be kept nearly uniform by inter- 
crossing ; so that many individuals will go on simultaneously 
changing, and the whole amount of modification at each stage will 


not be due to descent from a single parent. To illustrate what 
I mean : our English race-horses differ from the horses of every 
other breed ; but they do not owe their difference and superiority 
to descent from aay single pair, but to continued care in the 
selecting and training of many individuals during each generation. 

Before discussing the three classes of facts, which I have selected 
as presenting the greatest amount of difficulty on the theory of 
" single centres of creation," I mast say a few words on the meaus 
of dispersal. 

Means of Dispersal. 

Sir C. Lyell and other authors have ably treated this subject. 
I can give here only the briefest abstract of the more important facts. 
Change of climate must have had a powerful influence on migration. 
A region now impassable to certain organisms from the nature of 
its climate, might have been a high road for migration, when the 
climate was different. I shall, however, presently have to discuss 
this branch of the subject in some detail. Changes of level in the 
land must also have been highly influential : a narrow isthmus 
now separates two marine faunas ; submerge it, or let it formerly 
have been submerged, and the two faunas will now blend together, 
or may formerly have blended. Where the sea now extends, land 
may at a former period have connected islands or possibly even 
continents together, and thus have allowed terrestrial productions 
to pass from one to the other. No geologist disputes that great 
mutations of level have occurred within the period of existing 
organisms. Edward Forbes insisted that all the islands in the 
Atlantic must have been recently connected with Europe or Africa, 
.and Europe likewise with America. Other authors have thus 
hypothetically bridged over every ocean, and united almost every 
island to some mainland. If indeed the arguments used by Forbes 
are to be trusted, it must be admitted that scarcely a single island 
exists which has not recently been united to some continent. This 
view cuts the Gordian knot of the dispersal of the same species to 
the most distant points, and removes many a difficulty ; but to the 
best of my judgment we are not authorized in admitting such 
enormous geographical changes within the period of existing species. 
It seems to me that we have abundant evidence of great oscillations 
in the level of the land or sea ; but not of such vast changes in the 
position and extension of our continents, as to have united them 
within the recent period to each other and to the several intervening 
oceanic islands. I freely admit the former existence of many islands, 
now buried beneath the sea^ which msyhave served as halting- 

Y 2 

Stale Hisionca! ai.o 
Ki^.tur'..! Hi^Torv Society, 


places for plants and for many animals during their migration. In 
the coral-producing oceans such sunken islands are now marked by 
rings of coral or atolls standing over them . Whenever it is fully 
admitted, as it will some day be, that each species has proceeded 
from a single birthplace, and when in the course of time we know 
something definite about the means of distribution, we shall be 
enabled to speculate with security on the former extension of the 
land. But I do not believe that it will ever be proved that within 
the recent period most of our continents which now stand quite 
separate, have been continuously, or almost continuously united 
with each other, and with the many existing oceanic islands. 
Several facts in distribution, — such as the great difference in the 
marine faunas on the opposite sides of almost every continent, — 
the close relation of the tertiary inhabitants of several lands and 
even seas to their present inhabitants, — the degree of affinity be- 
tween the mammals inhabiting islands with those of the nearest 
continent, being in part determined (as we shall hereafter see) by 
the depth of the intervening ocean, — these and other such facts are 
opposed to the admission of such prodigious geographical revolutions 
within the recent period, as are necessary on the view advanced by 
Forbes and admitted by his followers. The nature and relative pro- 
portions of the inhabitants of oceanic islands are likewise opposed 
to the belief of their former continuity with continents. Nor does 
the almost universally volcanic composition of such islands favour 
the admission that they are the wrecks of sunken continents ; — it 
they had originally existed as continental mountain-ranges, some at 
least of the islands would have been formed, like other mountain- 
summits, of granite, metamorphic schists, old fossiliferoas and other 
rocks, instead of consisting of mere piles of volcanic matter. 

I must now say a few words on what are called accidental means, 
but which more properly should be called occasional means of dis- 
tribution. I shall here confine myself to plants. In botani<;al 
works, this or that plant is often stated to be ill adapted for wide 
dissemination ; but the greater or less facilities for transport across 
the sea may be said to be almost wholly unknown. Until I tried, 
with Mr. Berkeley's aid, a few experiments, it was not even known 
how far seeds could resist the injurious action of sea- water. To 
my surprise I found that out of 87 kinds, 64 germinated after an 
immersion of 28 days, and a few survived an immersion of 137 
days. It deserves notice that certain orders were far more injured 
than others : nine Leguminosje were tried, and, with one exception, 
they resisted the salt-water badly ; seven species of the allied orders, 
Hydrophyllaceai and PciemciniaceEe, were all killed by a month's 


immersion. I^or convenience' sake I chiefly tried small seeds, 
without the capsule or fruit ; and as all of these sank in a few days, 
they could not have been floated across wide spaces of the sea, 
whether or not they were injured by the salt-water. Afterwards 
I tried some larger fruits, capsules, &c., and some of these floated 
for a long time. It is well known what a diifeiKence there is in the 
buoyancy of green and seasoned timber ; and it occurred to me that 
Hoods would often wash into the sea dried plants or branches with 
seed-capsules or fruit attached to them. Hence I was led to dry the 
stems and branches of 94 plants with ripe fruit, and to place them 
on sea-water. The majority sank quickly, but some which, whilst 
green, floated for a very short time, when dried floated much longer ; 
for instance, ripe hazel-nuts sank immediately, but when dried 
they floated for 90 days, and afterwards when planted germinated ; 
an asparagus-plant with ripe berries floated for 23 days, when dried 
it floated for 85 days, and the seeds afterwards germinated ; the 
ripe seeds of Helosciadium sank in two days, when dried they 
floated for above 90 days, and afterwards germinated. Altogether, 
out of the 94 dried plants, 18 floated for above 28 days ; and 
some of the 18 floated for a very much longer period. So that as 
-^^ kinds of seeds germinated after an immersion of 28 days ; and 
as if distinct species with ripe fruit (but not all the same species 
as in the foregoing experiment) floated, after being dried, for above 
28 days, we may conclude, as far as anything can be inferred from 
these scanty facts, that the seeds of ^J-^q kmds of plants of any 
country might be floated by sea-currents during 28 days, and would 
retain their power of germination. In Johnston's Physical Atlas, 
the average rate of the several Atlantic currents is 33 miles per 
diem (some currents running at the rate of 60 miles per diem) ; on 
this average, the seeds of -^^ plants belonging to one country 
might be floated across 924 miles of sea to another country, and 
when stranded, if blown by an inland gale to a favourable spot, 
would germinate. 

Subsequently to my experiments, M. Martens tried similar ones, 
but in a much better manner, for he placed the seeds in a box in 
the actual sea, so that they were alternately wet and exposed to the 
air like really floating plants. He tried 98 seeds, mostly difi'erent 
from mine ; but he chose many large fruits and likewise seeds from 
plants which live near the sea ; and this would have favoured both 
the average length of their flotation and their resistance to the 
injurious action of the salt-water. On the other hand, he did not 
previously dry the plants or branches with the fruit ; and this, as 
ire have seen, would have caused some of them to have fi<.)ated 


much longer. The result was that ^ of his seeds of different 
kinds floati^d for 42 days, and were then capable of germination. 
But I do not doubt that plants exposed to the waves would fioiit 
for a less time than those protected from violent movement as in 
our experiments. Therefore it would perhaps be safer to assume 
that tlie seeds of about JJ'q plants of a flora, after having been 
dried, could be floated across a space of sea 900 miles in width, and 
would then germinate. The fact of the larger fruits often floating 
longer than the small, is interesting ; as plants with large seeds or 
fruit which, as Alph. de CandoUe has shown, generally have re- 
stricted ranges, could hardly be transported by any other means. 

Seeds may be occasionally transported in another manner. Drift 
timber is thrown up on most islands, even on those in the midst 
of the widest oceans ; and the natives of the coral-islands in the 
Pacific procure stones for their tools, solely from the roots of drifted 
trees, these stones being a valuable royal tax. I find that when 
irregularly shaped stones are embedded in tlie roots of trees, small 
parcels of earth are frequently enclosed in their interstices and 
behind them, — so perfectly that not a particle could be washed away 
during the longest transport : out of one small portion of earth 
thus completely enclosed by the roots of an oak about 50 years old, 
three dicotyledonous plants germinated : I am certain of the accu- 
racy of this observation. Again, I can show that the carcases of 
birds, when floating on the sea, sometimes escape being immediately 
devoured : and many kinds of seeds in the crops of floating birds 
long retain their vitality : peas and vetches, for instance, are killed 
by even a few days' immersion in sea-water ; but some taken out 
of the crop of a pigeon, which had floated on artificial sea-water for 
30 days, to my surprise nearly all germinated. 

Living birds can hardly fail to be highly effective agents in the 
transportation of seeds. I could give many facts showing how 
frequently birds of many kinds are blown by gales to vast distances 
across the ocean. We may safely assume that under such circum- 
stances their rate of flight would often be 35 miles an hour ; and 
some authors have given a far higher estimate. I have never seen 
an instance of nutritious seeds passing through the intestines of 
a bird ; but hard seeds of fruit pass uninjured through even the 
digestive organs of a turkey. In the course of two months, I picked 
up in my garden 12 kinds of seeds, out of the excrement of small 
birds, and these seemed perfect, and some of them, which were 
tried, germinated. But the following fact is more important : the 
crops of birds do not secrete gastric juice, and do not, as I know by 
trial, injure in the least the germination of seeds ; now, after a bird 


has found and dcovured a large sup])ly of food, it is positively 
asserted that all the grains do not pass into the gizzard for twelve 
or even eighteen hours. A bird in this interval might easily bo 
blown to the distance of 500 miles, and hawks are known to look 
out for tired birds, and the contents of their torn crops might thus 
readily get scattered. Some hawks and owls bolt their prey whole, 
and, after an interval of from twelve to twenty hours, disgorge 
pellets, which, as I know from experiments made in the Zoological 
Gardens, include seeds capable of germination. Some seeds of the 
oat, wheat, millet, canary, hemp, clover, and beet germinated aftei 
having been from twelve to twenty-one hours in the stomachs of 
different birds of prey; and two seeds of beet grew after having 
b^en thus retained for two days and fourteen hours. Fresh-water 
fish, I find, eat seeds of many land and water plants: fish are 
frequently devoured by birds, and thus the seeds might be trans- 
ported from place to place. I forced many kinds of seeds into the 
stomachs of dead fish, and then gave their bodies to fishing-eagles, 
stoi'ks, and pelicans ; these birds, after an interval of many hours, 
either rejected the seeds in pellets or passed them in their excre- 
ment ; and several of these seeds retained the power of germination. 
Certain seeds, however, were always killed by this process. 

Locusts are sometimes blown to great distances from the land ; I 
myself caught one 370 miles from the coast of Africa, and have 
heard of others caught at greater distances. The Rev. R. T. Lowe 
informed Sir C. Lyell that in November 1844 swarms of locusts 
visited the island of Madeira. They were in countless numbers, as 
thick as the flakes of snow in the heaviest snowstorm, and extended 
upwards as far as could be seen with a telescope. During two or 
three days they slowly careered rcand and roimd in an immense 
elli|)se, at least five or six miles in diameter, and at night alighted 
on the taller trees, which were completely coated with them. They 
then disappeared over the ' sea, as suddenly as they had appeared, 
and have not since visited the island. Now, in parts of Natal it is 
believed by some farmers, though on insufiicient evidence, that 
injurious seeds are introduced into their grass-land in the dung left 
by the great flights of locusts which often visit that country. In 
consequence of this belief Mr. Weale sent me in a letter a small 
packet of the dried pellets, out of which I extracted under the 
microscope several secda, and raised from them seven grass plants, 
belonging to two species, of two genera. Hence a swarm of locusts, 
such as that which visited Madeira, might readily be the means of 
introducing several kinds of plants into an island lying far from Ihe 


Although the beaks and feet of birds are generally clean, earth 
sometimes adheres to them : in one case I removed sixty-one grains, 
and in another case twenty-two grains of dry argillaceous earth 
from the foot of a partridge, and in the earth there was a pebble as 
large as the seed of a vetch. Here is a better case : the leg of a 
woodcock was sent to me by a friend, with a little cake of dry earth 
attached to the shank, weighing only nine grains ; and this con- 
tained a seed of the toad-rush (Juncus bufonius) which germinated 
and flowered. Mr. Swaysland, of Brighton, who during the last 
forty years has paid close attention to our migratory birds, informs 
me that he has often shot wagtails (Motacillas), wheatears, and whin- 
chats (Saxicolas), on their first arrival on our shores, before they 
had alighted ; and he has several times noticed little cakes of earth 
attached to their feet. Many facts could be given showing how 
generally soil is charged with seeds. For instance. Prof. Newton 
sent me the leg of a red-legged partridge (Caccabis rufa) which had 
been wounded and could not fly, with a ball of hard earth adhering 
to it, and weighing six and a half ounces. The earth had been 
kept for three years, but when broken, watered and placed under a 
bell glass, no less than 82 plants sprung from it : these consisted of 
12 monocotyledons, including the common oat, and at least one 
kind of grass, and of 70 dicotyledons, which consisted, judging from 
the young leaves, of at least three distinct species. With such facts 
before us, can we doubt that the many birds which are annually 
blown by gales across great spaces of ocean, and which annually 
migrate — for instance, the millious of quails across the Mediterra- 
nean — must occasionally transport a few seeds embedded in dirt 
adhering to their feet or beaks ? But I shall have to recur to this 

As icebergs are known to be sometimes loaded with earth and 
stones, and have even carried brushwood, bones, and the nest of a 
land-bird, it can hardly be doubted that they must occasionally, as 
suggested by Lyell, have transported seeds from one part to another 
of the arctic and antarctic regions; and during the Glacial period 
from one part of the now temperate regions to another. In the 
Azores, from the large number of plants common to Europe, in com- 
parison with the species on the other islands of the Atlantic, which 
stand nearer to the mainland, and (as remarked by Mr. H. C. 
Watson) from their somewhat northern character in comparison 
with the latitude, I suspected that these islands had been partly 
stocked by ice-borne seeds, during the Glacial epoch. At my request 
Sir C. Lyell wrote to M. Hartung to inquire whether he had 
observed erratic boulders on these islands, and he answered that he 


had found large fragments of granite and other rocks, which do not 
occur in the archipelago. Hence we may safely infer that icebergs 
formerly landed their rocky burthens on the shores of these mid- 
ocean islands, auvl it is at least possible that they may have brought 
thither some few seeds of northern plants. 

Considering that these several means of transport, and that other 
means, which without doubt remain to be discov^ered, have been in 
action year after year for tens of thousands of years, it would, I 
think, be a marvellous fact if many plants had not thus become 
widely transported. These means of transport are sometimes called 
accidental, but this is not strictly correct : the currents of the sea 
are not accidental, nor is the direction of prevalent gales of wind, 
it should be observed that scarcely any means of transport would 
carry seeds for very great distances : for seeds do not retain their 
vitality when exposed for a great length of time to the action of 
sea-water; nor could they be long cari'icd in the crops or intestines 
of birds. These means, however, would suffice for occasional trans- 
port across tracts of sea some hundred miles in breadth, or from 
island to island, or from a continent to a neighbouring island, but 
not from one distant continent to another. The floras of distant 
continents would not by such means become mingled ; but would 
remain as distinct as they now are. The currents, from their 
course, would never bring seeds from North America to Britain, 
though they might and do bring seeds from the West Indies to our 
western shores, where, if not killed by their very long immersion in 
salt water, they could not endure our climate. Almost every year, 
one or two land-birds are blown across the whole Atlantic Ocean, 
from North America to the western shores of Ireland and England ; 
but seeds could be transported by these rare wanderers only by 
one means, namely, by dirt adhering to their feet or beaks, w^hich is 
in itself a rare accident. Even in this case, how small would be the 
chance of a seed falling on favourable soil, and coming to maturity I 
But it would be a great error to argue that because a well-stocked 
island, like Great Britain, has not, as far as is known (and it w^ould 
be very difficult to prove this), received within the last few centu- 
ries, through occasional means of transport, immigrants from Europe 
or any other continent, that a poorly-stocked island, though stand- 
ing more remote from the mainland, would not receive colonists by 
similar means. Out of a hundred kinds of seeds or animals trans- 
ported to an island, even if far less well- stocked than Britain, per- 
haps not more than one would be so well fitted to its new home, as 
to become naturalised. But this is no valid argument against what 
wou^d be elTected by occasional means of transj^ort, during tlie Iod^ 


lapse of geological time, whilst the island was being upheaved, and 
before it had become fully stocked with inhabitants. On almost 
bare land, with few or no destructive insects or cirds living there, 
nearly every seed which chanced to aiTive, if fitted, for the climate, 
would germinate and survive. 

Dispersal during the Glacial Perijd. 

The identity of many plants and animals, on mountain-summits, 
separated from each other by hundreds of miles of lowlands, where 
Alpine species could not possibly exist, is one of the most striking 
cases known of the same species living at distant points, without 
the apparent jwssibility of their having migrated from one point 
to the other. It is indeed a remarkable fact to see so many plants 
of the same species living on the snowy regions of the Alps or 
P^n-enecs, and in the extreme northern parts of Europe ; but it is 
far more remarkable, that the plants on the White Mountains, in* 
the United States of America, are all the same with those of 
Labrador, and nearly all the same, as we hear from Asa Gray, with 
those on the loftiest mountains of Europe. Even as long ago as 
1747, such facts led Gmelin to conclude that the same species must 
have been independently created at many distinct points ; and we 
might have remained in this same belief, had not Agassiz and 
others called vivid attention to the Glacial period, which, as we 
shall immediately see, affords a simple explanation of these facts. 
We have evidence of almost every conceivable kind, organic and 
inorganic, that, within a very recent geological period, central 
Europe and North America suffered under an arctic climate. The 
ruins of a house burnt by fire do not tell their tale more plainly 
than do the mountains of Scotland and Wales, with their scored 
flanks, polished surfaces, and perched boulders, of the icy streams 
with which their valleys were lately filled. So greatly has the 
climate of Europe changed, that in Northern Italy, gigantic moraines, 
left by old glaciers, are now clothed by the vine and maize. Through- 
out a large part of the United States, erratic boulders and scored 
rocks plainly reveal a former cold period. 

The former influence of the glacial climate on the distribution of 
the inhabitants of Europe, as explained by Edward Forbes, is sub- 
stantially as follows. But we shall follow the changes more readily, 
by supposing a new glacial period slowly to come on, and then 
pass away, as formerly occurred. As the cold came on, and as 
each more southern zone became fitted for the inhabitants of the 
north, these would take the plac<cs cf the former inhabitants of 
the temperate regions. The latter, at the same time, would travel 


fui^her and further southward, unless they were stopped by barriers, 
in which case they would perish. The mountains would become 
covered with snow and ice, and their former Alpine inhabitants 
would descend to the plains. By the time that the cold had reached 
its maximum, we should have an arctic fauna and flora, covering 
th3 central parts of Europe, as far south as the Alps and Pyrenees, 
and even stretching into Spain. The now temperate regions of the 
United States would likewise be covered by arctic plants and ani- 
mals and these would be nearly the same with those of Europe ; for 
the present circumpolar inhabitants, which we suppose to have every- 
where travelled southward, are remarkably uniform round the world. 

As the warmth returned, the arctic forms would retreat north- 
ward, closely followed up in their retreat by the productions of the 
more temperate regions. And as the snow melted from the bases 
of the mountains, the arctic forms would seize on the cleared 
and thawed ground, always ascending, as the warmth increased and 
the snow still further disappeared, higher and higher, whilst their 
brethren were pursuing their northern journey. Hence, when the 
warmth had fully returned, the same species, which had lately 
lived together on the European and North American lowlands, 
would again be found in the arctic regions of the Old and New 
Worlds, and on many isolated mountain-summits far distant from 
each other. 

Thus we can understand the identity of many plants at points 
so immensely remote as the mountains of the United States and 
those of Europe. We can thus also understand the fact that the 
Alpine plants of each mountain- range are more especially related 
to the arctic forms living due north or nearly due north of them : 
for the first migration when the cold came on, and the re-migratior 
on the returning warmth, would generally have been due south and 
north. The Alpine plants, for example, of Scotland, as remarked 
by Mr. H. C. Watson, and those of the Pyrenees, as remarked by 
Eamond, are more especially allied to the plants of northern Scandi- 
navia ; those of the United States to Labrador ; those of the moun- 
tains of Siberia to the arctic regions of that country. These views, 
grounded as they are on the perfectly well-ascertained occurrence of 
a former Glacial period, seem to me to explain in so satisfactory a 
manner the present distribution of the Alpine and Arctic produc- 
tions of Europe and America, that when in other regions we find 
the same species on distant mountain-summits, we may almost 
conclude, without other evidence, that a colder climate formerly 
permitted their migration across the intervening lowlands, now 
become too warm for their existence. 


As the arctic forms moved first southward and aftervvra'ds back- 
wards to the north, in unison with the changing climate, they will 
not have been exposed during their long migrations to any great 
diversity of temperature; and as they all migrated in a body 
together, their mutual relations will not have been much disturbed. 
Hence, in accordance wich the principles inculcated in this volume, 
these forms will not have been liable to much modification. But 
with the Alpine productions, left isolated from the moment of the 
returning warmth, first at the bases and ultimately on the summits 
of the mountains, the case will have been somewhat different ; for 
it is not likely that all the same arctic species will have been left 
on mountain-ranges far distant from each other, and have survived 
there ever since ; they will also in all probability, have become 
mingled with ancient Alpine species, which must liave existed on 
the mountains before the commencement of the Glacial epoch, and 
which during the coldest period will have been ten:)porarily driven 
down to the plains ; they will, also, have been subsequently ex- 
posed to somewhat different climatal influences. Their mutual rela- 
tions will thus have been in some degree disturbed; consequently 
they will have been liable to modification ; and they have been 
modified ; for if we compare the present Alpine plants and animals 
of the several great European mountain-ranges one with another, 
though many of the species remain identically the same, some 
exist as varieties, some as doubtful forms or sub-species, and 
some as distinct yet closely allied species representing each other 
on the several ranges. 

In the foregoing illustration I have assumed that at the com- 
mencement of our imaginary Glacial period, the arctic productions 
were as uniform round the polar regions as they are at the present 
day. But it is also necessary to assume that many sub-arctic and 
some few temperate forms were the same round the world, for 
some of the species which now exist on the lower mountain-slopes 
and on the plains of North America and Europe are the same; 
and it may be asked how I account for this degree of uniformity 
in the sub-arctic and temperate forms round the world, at the 
commencement of the real Glacial period. At the j^resent day, 
the sub-arctic and northern temperate productions of the Old 
and New Worlds are separated from each other by the whok 
Atlantic Ocean and by the northern part of the Pacific. During 
the Glacial period, when the inhabitants of the Old and Ne\/ 
Worlds lived farther southwards than they do at present, they 
must have been still more completely separated from each othej 
by wider spaces of ocean ; so that it may well be asl^ed now tno 


same species could then or previously Lave entered the two con- 
tinents. The explanation, I believe, lies in the nature of the 
climate before the commencement of the Glacial period. At this, 
the newer Pliocene period, the majority of the inhabitants of the 
world were specifically the same as now, and we have good reason 
to believe that the climate was warmer than at the present day 
Hence we may suppose that the organisms which now live undei 
latitude 60°, lived during the Pliocene period farther north under the 
Polar Circle, in latitude 66°-67° ; and that the present arctic pro- 
ductions then lived on the broken land still nearer to the pole. 
Now, if we look at a terrestrial globe, we see under the Polar Circle 
that there is almost continuous land from western Europe, through 
Siberia, to eastern America. And this continuity of the circum- 
polar land, with the consequent freedom under a more favourable 
climate for intermigration, will account for the supposed uniformity 
of the sub-arctic and temperate productions of the Old and New 
Worlds, at a period anterior to the Glacial epoch. 

Believing, from reasons before alluded to, that our continents 
have long remained in nearly the same relative position, though 
subjected to great oscillations of level, I am strongly inclined to 
extend the above view, and to infer that during some still earlier 
and still warmer period, such as the older Pliocene period, a large 
number of the same plants and animals inhabited the almost 
continuous circumpolar land ; and that these plants and animals, 
both in the Old and New Worlds, began slowly to migrate south- 
wards as the climate became less warm, long before the commence- 
ment of the Glacial period. We now see, as I believe, their 
descendants, mostly in a modified condition, in the central parts 
of Europe and the United States. On this view we can under- 
stand the relationship with very little identity, between the pro- 
ductions of North America and Europe, — a relationship which is 
highly remarkable, considering the distance of the two areas, and 
their separation by the whole Atlantic Ocean. We can further 
understand the singular fact remarked on by several observers 
that the productions of Europe and America during the later 
tertiary stages were more closely related to each other than they 
are at the present time ; for during these warmer periods the 
northern parts of the Old and New Worlds will have been almost 
continuously united by land, serving as a bridge, since rendered 
impassable by cold, for the intermigration of their inhabitants. 

During the slowly decreasing warmth of the Pliocene period, as 
Boon as the species in common, which inhabited the New and Old 
Worlds, migrated south of the Polar Circle, they will have been 


completely cut oS from each other. This separation, as far as the 
more temperate productions are concerned, must have taken place 
long ages ago. As the plants and animals migrated southward, 
they will have hecome mingled in the one great region with the 
native American productions, and would have had to compete 
with them ; and in the other great region, with those of the Old 
World. Consequently we have here everything favourahle for 
much modification, — for far more modification than with the 
Alpine productions, left isolated, within a much more recent 
period, on the several mountain-ranges and on the arctic lands 
of Europe and N. America. Hence it has come, that when we 
compare the now living productions of the temperate regions of 
the New and Old Worlds, we find very few identical species (though 
Asa Gray has lately shown that more plants are identical than was 
formerly supposed), hut we find in every great class many forms, 
which some naturalists rank as geographical races, and others as dis- 
tinct species ; and a host of closely allied or representative forms 
which are ranked hy all naturalists as specifically distinct. 

As on the land, so in the waters of the sea, a slow southern 
migration of a marine fauna, which, during the Pliocene or even a 
somewhat earlier period, was nearly uniform along the continuous 
shores of the Polar Circle, will account, on the theory of modifica- 
tion, for many closely allied forms now living in marine areas com- 
pletely sundered. Thus, I think, we can understand the presence 
of some closely allied, still existing and extinct tertiary forms, on 
the eastern and western shores of temperate North America ; and 
the still more striking fact of many closely allied crustaceans (as 
described in Dana's admirable work), some fish and other marine 
animals, inhabiting the Mediterranean and the seas of Japan, — 
these two areas being now completely separated by the breadth of a 
whole continent and by wide spaces of ocean. 

These cases of close relationship in species either now or formerly 
inhabiting the seas on the eastern and western shores of North 
America, the Mediterranean and Japan, and the temperate lands 
of North America and Europe, are inexplicable on the theory of 
creation. We cannot maintain that such species have been created 
alike, in correspondence with the nearly similar physical conditions 
of the areas ; for if we compare, for instance, certain parts of South 
America with parts of South Africa or Australia, we see countries 
closely similar in all their physical conditions, with their inhal>- 
itants utterly dissimilar. 


Alternate Qlacial Periods in the North and South. 

■ But we must return to onr more immediate subject. 1 arc con- 
vinced that Forbes's view may be largely extended. In Europe we 
meet with the plainest evidence of the Glacial period, from the 
western shores of Britain to the Oural range, and southward to the 
Pyrenees. We may infer from the frozen mammals and nature 
of the mountain vegetation, that Siberia was similarly affected. In 
the Lebanon, according to Dr. Hooker, perpetual snow formerly 
covered the central axis, and fed glaciers which rolled 4000 feet 
down the valleys. The same observer has recently found great 
moraines at a low level on the Atlas range in N. Africa. Along 
the Himalaya, at points 900 miles apart, glaciers have left the 
marks of their former low descent ; and in Sikkim, Dr. Hooker 
saw maize growing on ancient and gigantic moraines. Southward 
of the Asiatic continent, on the opposite side of the equator, we 
know, from the excellent researches of Dr. J. Haast and Dr. Hector, 
that in New Zealand immense glaciers formerly descended to a 
low level ; and the same plants found by Dr. Hooker on widely 
separated mountains in this island tell the same story of a former 
cold period. From facts communicated to me by the Rev. W. B. 
Clarke, it appears aho that there are traces of former glacial action 
on the mountains of the south-eastern corner of Australia. 

Looking to America ; in the northern half, ice-borne fragments of 
rock have been observed on the eastern side of the continent, as far 
south as lat. 36°-37°, and on the shores of the Pacific, v;here the 
climate is now so different, as far south as lat. 46°. Erratic boulders 
have, also, been noticed on the Rocky Mountains. In the Cor- 
dillera of South America, nearly under the equator, glaciers once 
extended far below their present level. In Central Chile I ex- 
amined a vast mound of detritus with great boulders, crossing the 
Portillo valley, which there can hardly be a doubt once formed a 
huge moraine; and Mr. D. Forbes informs me that he found in 
various parts of the Cordillera, from lat. 13° to 30° S., at about the 
height of 12,000 feet, deeply-furrowed rocks, resembling those with 
which he was familiar in Norway, and likewise great masses of 
detritus, including grooved pebbles. Along this whole space of the 
Cordillera true glaciers do not now exist even at much more con- 
siderable heights. Farther south on both sides of the continent, 
from lat. 41° to the southernmost extremity, we have the clearest 
evidence of former glacial action, in numerous immense boulders 
transported far from their parent source. 

From these several facts, namely from the glacial action having 


extended all round the nortiiern and southern hemispheres — from 
the period having been in a geological sense recent in both hemi- 
spheres—from its having lasted in both during a great length of 
time, as may be inferred from the amount of work effected — and 
lastly from glaciers having recently descended to a low level along; 
the whole line of the Cordillera, it at one time appeared to me that 
we could not avoid the conclusion that the temperature of the 
whole world had been simultaneously lowered during the Glacial 
period. But now Mr. CroU, in a series of admirable memoirs, has 
attempted to show that a glacial condition of climate is the result 
of various physical causes, brought into operation by an increase in 
the eccentricity of the earth's orbit. All these causes tend towards 
the saine end ; but the most powerful appears to be the indirect 
influence of the eccentricity of the orbit upon oceanic currents. 
According to Mr. Croll, cold periods regularly recur every ten or 
fil'teen thousand years; and these at long intervals are extremely 
severe, owing to certain contingencies, of which the most important, 
as Sir C. Lyell has shown, is the relative position of the land and 
water. Mr. Croll believes that the last great Glacial period occurred 
about 240,000 years ago, and endured with slight alterations of 
climate for about 1G0,000 years. With respect to more ancient 
Glacial periods, several geologists are convinced from direct evidence 
that such occurred during the Miocene and Eocene formations, not 
to mention still more ancient formations. But the most important 
result for us, arrived at by Mr. Croll, is that whenever the northern 
hemisphere passes through a cold period, the temperature of the 
southern hemisphere is actually raised, with the winters rendered 
much milder, chiefly through changes in the direction of the ocean- 
currents. So conversely it will be with the northern hemisphere, 
whilst the southern passes though a glacial period. This conclusion 
throws so much light on geographical distribution that I am 
strongly inclined to trust in it ; but I will first give the facts, which 
demand an explanation. 

In South America, Dr. Hooker has shown that besides many 
closelj'' allied species, between forty and fifty of the flowering plants 
of Tierra del Fuego, forming no inconsiderable part of its scanty 
flora, are common to North America and Europe, enormously 
remote as these areas in opposite hemispheres are from each other. 
On the lofty mountains of equatorial America a host of peculiar 
Bpecies belonging to European genera occur. On the Organ moun- 
tains of Brazil, some few temperate European, some Antarctic, and 
some Andean genera were found by Gardner, which do not exist 
In the low intervening hot countries. On the Silla of Caraccas» 


tlie illustrious Humboldt long ago found species belonging to j^enera 
characteristic of the Cordillera. 

In Africa, several forms characteristic of Europe and some few 
representatives of the flora of the Cape of Good Hope occur on the 
mountains of Abyssinia. At the Cape of Good Hope a very few 
European species, believed not to have been introduced by man, 
and on the mountains several representative European forms are 
found, which have not been discovered in the intertropical parts of 
Africa. Dr. Hooker has also lately shown that several of the plants 
living on the upper parts of the lofty island of Fernando Po and on 
the neighbouring Cameroon mountains, in the Gulf of Guinea, are 
closely related to those on the mountains of Abyssinia, and likewise 
to those of temperate Europe. It now also appears, as I hear from 
Dr. Hooker, that some of these same temperate plants have been 
discovered by the Rev. R. T. Lowe on the mountains of the Cape 
Verde islands. This extension of the same temperate forms, almost 
under the equator, across the whole continent of Africa and to 
the mountains of the Caps Verde archipelago, is one of the most 
astonishing facts ever recorded in the distribution of plants. 

On the Himalaya, and on the isolated mountain-ranges of the 
peninsula of India, on the heights of Ceylon, and on the volcanic 
cones of Java, many plants occur, either identically the same or 
representing each other, and at the same time representing plants 
of Europe, not found in the intervening hot lowlands. A list of 
the genera of plants collected on the loftier peaks of Java, raises 
a picture of a collection made on a hillock in EurojDe ! Still more 
striking is the fact that peculiar Australian forms are represented 
by certain plants growing on the summits of the mountains of 
Borneo. Some of these Australian forms, as I hear from Dr. Hooker, 
extend along the heights of the peninsula of Malacca, and are 
thinly scattered on the one hand over India, and on the other hand 
as far north' as Japan. 

On the southern mountains of Australia, Dr. F. Mtiller has 
discovered several European species ; other species, not introduced 
by man, occur on the lowlands ; and a long list can be given, as I 
am informed by Dr. Hooker, of European genera, found in Australia, 
but not in the intermediate torrid regions. In the admirable 
* Introduction to the Flora of New Zealand,' by Dr. Hooker, analo- 
gous and striking facts are given in regard to the plants of that 
large island. Hence we see that certain plants growing on the more 
lofty mountains of the tropics in all parts of the world, and on the 
t'jmperate plains of the north and south, are either the same species 
or varieties of the same species. It should, however, be observed 



that these plants are not strictly arctic forms ; for, as Mr. H. C. 
Watson has remarked, " in receding from polar towards equatorial 
latitudes, the Alpine or mountain floras really become less and less 
Arctic." Besides these identical and closely allied forms, many 
species inhabiting the same widely sundered areas, belong to genera 
not now found in the intermediate tropical lowlands. 

These brief remarks apply to plants alone ; but some few analogous 
facts could be given in regard to terrestrial animals. In marine 
productions, similar cases likewise occur ; as an example, I may 
(piote a statement by the highest authority. Prof. Dana, that " it is 
certainly a wonderful fact that New Zealand should have a closer 
resemblance in its Crustacea to Great Britain, its antipode, than to 
any other part of the world." Sir J. Richardson, also, speaks ot 
the reappearance on the shores of New Zealand, Tasmania, &c., 
of northern forms of fish. Dr. Hooker informs me that twenty- 
five species of Alg£e are common to New Zealand and to Europe, 
but have not been found in the intermediate tropical seas. 

From the foregoing facts, namely, the presence of temperate forms 
on the highlands across the whole of equatorial Africa, and along 
the Peninsula of India, to Ceylon and the Malay Archipelago, and 
in a less well-marked manner across the wide expanse of tropical 
South America, it appears almost certain that at some former 
period, no doubt during the most severe part of a Glacial period, 
the lowlands of these great continents were everj'-where tenanted 
ander the equator by a considerable number of temperate forms. 
At this period the equatorial climate at the level of the sea was 
probably about the same with that now experienced at the height 
of from five to six thousand feet under the same latitude, or 
perhaps even rather cooler. During this, the coldest period, the 
lowlands under the equator must have been clothed with a mingled 
tropical and temperate vegetation, like that described by Hooker as 
growing luxuriantly at the height of from four to five thousand feet 
on the lower slopes of the Himalaya, but with perhaps a still 
greater preponderance of temperate forms. So again in the moun- 
tainous island of Fernando Po, in the Gulf of Guinea, Mr. Mann 
found temperate European forms beginning to appear at the height 
of about five thousand feet. On the mountains of Panama, at the 
height of only two thousand feet. Dr. Seemann found the vegetation 
like that of Mexico, " with forms of the torrid zone harmoniously 
blended with those of the temperate." 

Now let us see whether Mr. Croll's conclusion that when the 
northern hemisphere suffered from the extreme cold of the great 
Glacial period, the southern hemisphere was actually warmer, throws 


any clear light on the present apparently inexplicable distribution of 
various organisms in the temperate parts of both hemispheres, and on 
the mountains . of the tropics. The Glacial period, as measured by 
years, must have been very long ; and when we remember over what 
vast spaces some naturalised plants and animals have spread within 
a few centuries, this period will have been ample for any amount of 
migration. As the cold became more and more intense, we know 
that Arctic forms invaded the temperate regions; and, from the 
iacts just given, there can hardly be a doubt that some of the more 
vigorous, dominant and widest-spreading temperate forms invaded 
the equatorial lowlands. The inhabitants of these hot lowlands 
would at the same time have migrated to the tropical and sub- 
tropical regions of the south, for the southern hemisphere was at this 
period warmer. On the decline of the Glacial period, as both hemi- 
spheres gradually recovered their former temperatures, the northern 
temperate forms living on the lowlands under the equator, would 
have been driven to their former homes or have been destroyed, 
being replaced by the equatorial forms returning from the south. 
Some, however, of the northern temperate forms would almost 
certainly have ascended any adjoining high land, where, if suflB- 
ciently lofty, they would have long survived like the Arctic forms 
on the mountains of Europe. They might have survived, even if 
the climate was not perfectly fitted for them, for the change of tem- 
perature must have been very slow, and plants undoubtedly possess a 
certain capacity for acclimatisation, as shown by their transmitting to 
their offspring different constitutional powers of resisting heat and cold. 
In the regular course of events the southern hemisphere would in 
its turn be subjected to a severe Glacial period, with the northern 
hemisphere rendered warmer ; and then the southern temperate 
forms would invade the equatorial lowlands. The northern forms 
which had before been left on the mountains would now descend 
and mingle with the southern forms. These latter, when the 
warmth returned, would return to their former homes, leaving some 
few species on the mountains, and carrying southward with them 
some of the northern temperate forms which had descended from 
their mountain fastnesses. Thus, we should have some few species 
identically the same in the northern and southern temperate zones 
and on the mountains of the intermediate tropical regions. But 
the species left during a long time on these mountains, or in opposite 
hemispheres, would have to compete with many new forms and 
would be exposed to somewhat different physical conditions ; hence 
they would be eminently liable to modification, and would generally 
now exist as varieties cr as representative species ; and this is the 

z 2 


case. We must, also, bear in mind the occurrence in both hemi- 
spheres of former Glacial periods ; for these will accoimt. in 
accordance with the same principles, for the many quite distinct 
species inhabiting the same widely separated areas, and belonging to 
genera not now found in the intermediate torrid zones. 

It is a remarkable fact stronf^ly insisted on by Hooker in regard 
to America, and by Alph. de Candolle in regard to Australia, that 
many more identical or slightly modified species have migrated from 
the north to the south, than in a reversed direction. We see, 
however, a few southern forms on the mountains of Borneo and 
Abyssinia. I suspect that this preponderant migration from the 
north to the south is due to the greater extent of land in the north, 
and to the northern forms having existed in their own homes in 
greater numbers, and having consequently been advanced through 
natural selection and competition to a higher stage of perfection, or 
dominating power, than the southern forms. And thus, when the 
two sets became commingled in the equatorial regions, during 
the alternations of the Glacial periods, the northern forms were the 
more powerful and were able to hold their places on the mountains, 
and afterwards to migrate southward with the southern forms ; but 
not so the southern in regard to the northern forms. In the 
same manner at the present day, we see that very many European 
productions cover the ground in La Plata, New Zealand, and to a 
lesser degree in Australia, and have beaten the natives ; whereas 
extremely few southern forms have become naturalised in any part 
of the northern hemisphere, though hides, wool, and other objects 
likely to carry seeds have been largely imported into Eui-ope during 
the last two or three centuries from La Plata, and during the last 
forty or fifty years from Australia. The Neilgherrie mountains in 
India, however, offer a partial exception ; for here, as I hear from 
Dr. Hooker, Australian forms are rapidly sowing themselves and 
becoming naturalised. Before the last great Glacial period, no 
doubt the intertropical mountains were stocked with endemic Alpine 
forms ; but these have almost everywhere yielded to the more 
dominant forms, generated in the larger areas and more efficient 
workshops of the north. In many islands the native productions 
are nearly equalled, or even outnumbered, by those which have 
become naturalised ; and this is the first stage towards their 
extinction. Mountains are islands on the land, and their inhabi- 
tants have yielded to those produced within the larger areas of the 
north, just in the same way as the inhabitants of real islands have 
everywhere yielded and are still yielding to continental forms 
naturalised through man's agency. 


The same principles apply to the di'Stribution of terresi.rial 
animals and of marine productions, in the northern and southern 
temperate zones, and on the intertropical mountains. "When, 
during the height of the Glacial period, the ocean-currents were 
widely different to what they now are, some of tlie inhabitants of 
the temperate seas might have reached the equator ; of these a few 
would perhaps at once be able to migrate southward, by keeping to 
the cooler currents, whilst others might remain and survive in the 
colder depths until the southern hemisphere was in its turn sub- 
jected to a glacial climate and permitted their further progress ; in 
nearly the same manner as, according to Forbes, isolated spaces 
inhabited by Arctic productions exist to the present day in the 
deeper parts of the northern temperate seas. 

I am far from supposing that all the difficulties in regard to th3 
distribution and affinities of the identical and allied species, which 
now live so widely separated in the north and south, and sometimes 
on the intermediate mountain-ranges, are removed on the viev/s 
above given. The exact lines of migration cannot be indicated. 
We cannot say why certain species and not others have migrated ; 
why certain species have been modified and have given rise to new 
forms, whilst others have remained unaltered. We cannot hope 
to explain such facts, until we can say why one species and not 
another becomes naturalised by man's agency in a foreign land ; 
why one species ranges twice or thrice as far, and is twice or thrice 
as common, as another species wnthin their own homes. 

Various special difficulties also remain to be solved ; for instance, 
the cccurrence, as shown by Dr. Hooker, of the same plants at points 
so enormously remote as Kerguelen Land, New Zealand, and Fuegia ; 
but icebergs, as suggested by Lyell, may have been concerned in 
their dispersal. 1'he existence at these and other distant points of 
ih-^ southern hemisphere, of species, which, though distinct, belong 
to genera exclusively confined to the south, is a more remarkable 
case. Some of these species are so distinct, that we cannot sup- 
pose that there has been time since the commencement of the last 
Glacial period for their migration and subsequent modification 
10 the necessary degree. The facts seem to indicate that distinct 
species belonging to the same genera have migrated in radiating 
lines from a common centre; and I am inclined to look in the 
southern, as in the northern hemisphere, to a former and warmer 
period, before the commencement of the last Glacial period, when 
the Antarctic lands, now covered with ice, supported a highly 
peculiar and isolated flora. It may be suspected that before this flora 
ivas exterminated during the last Glacial epoch, a few forms had 


been already widely dispersed to various points of the southern hemi- 
sphere by occasional means of transport, and by the aid as halting- 
places, of now sunken islands. Thus the southern shores of America, 
Australia, and New Zealand, may have become slightly tinted by 
the same peculiar forms of life. 

Sir C. Lyell in a striking passage has speculated, in language 
almost identical with mine, on the effects of great alternations of 
climate throughout the world on geographical distribution. And 
we have now^ seen that Mr. Croll's conclusion that successive Glacial 
periods in the one hemisphere coincide with warmer periods in the 
opposite hemisphere, together with the admission of the slow modifi- 
cation of species, explains a multitude of facts in the distribution of 
the same and of the allied forms of life in all parts of the globe. The 
living waters have flowed during one period from the north and 
during another from the south, and in both cases have reached the 
equator : but the stream of life has flowed with greater force from 
the north than in the opposite direction, and has consequently more 
freely inundated the south. As the tide leaves its drift in hori- 
zontal lines, rising higher on the shores where the tide rises highest, 
so have the living waters left their living drift on our mountain 
summits, in a line gently rising from the Arctic lowlands to a great 
altitude under the equator. The various beings thus left stranded 
may be compared with savage races of man, driven up and surviving 
ill the mountain fastnesses of almost every land, which serve as a 
record, full of interest to us, of the foimer inhabitants of tLs 
sun-oundmg lowlands. 



Geographical Distribution — continued. 

Distribution of fresh-water productions — On the inhabitants of oceanic 
islands — Absence of Batrachians and of terrestrial Mammals — On the 
relation of the inhabitants of islands to those of the nearest mainland — 
On colonisation from the nearest source with subsequent modification 
— Summary of the last and present chapter. 

Fresh-water Productions. 

As lakes and river-systems are separated from each other by barriers 
of land, it might have been thought that fresh-water productions 
would not have ranged widely within the same country, and as the 
sea is apparently a still more formidable barrier, that they would 
never have extended to distant countries. But the case is exactly 
the reverse. Not only have many fresh- water species, belonging to 
different classes, an enormous range, but allied species prevail in a 
remarkable manner throughout the world. When first collecting 
in the fresh waters of Brazil, I well remember feeling much sur- 
prise at the similarity of the fresh-water insects, shells, &c., and 
at the dissimilarity of the surrounding terrestrial beings, compared 
with those of Britain. 

But the wide ranging power of fresh-water productions can, I 
think, in most cases be explained by their having become fitted, in 
a manner highly useful to them, for short and frequent migrations 
from pond to pond, or from stream to stream within their own 
countries; and liability to wide dispersal would follow from this 
capacity as an almost necessary consequence. We can here consider 
only a few cases ; of these, some of the most difficult to explain 
arc presented by fish. It was formerly believed that the same 
fresh-water species never existed on two continents distant from 
each other. But Dr. Giinther has lately shown that the Galaxias 
attenuatus inhabits Tasmania, New Zealand, the Falkland Islands, 
and the mainland of South America. This is a wonderful case, and 
probably indicates dispersal from an Antarctic centre during a former 
Warm period. This case, however, is rendered in some degree less 


surprising by tlie species of this genus having the power of crossing 
hy some unknown means considerable spaces of open ocean : thus 
tli(!rc is one species common to New Zealand and to the Auckland 
Islands, though separated by a distance of about 230 miles. On 
the same continent fiesh- water fish often range widely, and as if 
c^pi'iciously ; for in two adjoining river-systems some of the species 
may be the same, and some wholly ditlerent. It is probable that 
they are occasionally transported by what may be called accidental 
means. Thus fishes still alive are not very rarely dropped at distant 
points by whirlwinds ; and it is known that the ova retain their 
vit,ality for a considerable time after removal from the water. 
Their dispersal may, however, be mainly attributed to changes in 
the level of the land within the recent period, causing rivers to flow 
into each other. Instances, also, could be given of this having 
occurred during floods, without any change of leveL The wide 
difference of the fish on the opposite sides of most mountain-ranges, 
which are continuous, and which consequently must from an early 
period have completely jireveiited the inosculation of the river- 
systems on the two sides, leads to the same conclusion. Some 
fresh-water fish belong to very ancient forms, and in such cases 
there will have been ample time for great geographical changes, and 
consequently time and means for much migration. Moreover Dr. 
Giinther has recently been led by several considerations to infer 
that with fishes the s:ime forms have a long endurance. Salt-water 
fish can with care be slowly accustomed to live in fresh water ; 
and, according to Valenciennes, there is hardly a single group ot 
which all the members are confined to fresh water, so that a marine 
species belonging to a fresh-water group might travel far along the 
shores of the sea, and could, it is probable, become adapted without 
much difficulty to the fresh waters of a distant land. 

Some species of fresh-water shells have very wide ranges, and 
allied species which, on our theory, are descended from a common 
parent, and must have proceeded from a single source, prevail 
throughout the world. ^Jlieir distribution at first perplexed me 
much, as their ova are not likely to be transported by birds ; and 
the ova, as well as the adults, are immediately killed by sea-water. 
I could not even understand how some naturalised species have 
spread rapidly throughout the same country. But two facts, which 
I have observed — and many others no doubt will be discovered — 
throw some light on this subject. When ducks suddenly emerge 
from a pond covered with duck-weed, I have twice seen these little 
plants adhering to their backs ; and it has happened to me, in 
removing a little duck-weed from one aquarium to a nether, that 1 


have umntentionally stocked the one with fresh-water shells from 
the other. But another agency is perhaps more effectual : I sus- 
pended the feet of a duck in an aquarium, where many ova of fresh- 
water shells w^ere hatching; and I found that numbers of the 
extremely minute and just-hatched shells crawled on the feet, and 
clung to them so firmly that when taken out of the water they 
could not be jarred off, though at a somewhat more advanced age 
they would voluntarily drop off. I'hese just-hatched molluscs, 
though aquatic in their nature, survived on the duck's feet, in 
damp air, from twelve to twenty hours ; and in this length of time 
a duck or heron might fly at least six or seven hundred miles, and 
if blown across the sea to an oceanic island, or to any other distant 
point, would be sure to alight on a pool or rivulet. Sir Charlea 
Lyell informs me that a Dytiscus has been caught with an Ancylus 
(a fresh-water shell like a limpet) firmly adhering to it ; and a 
water-beetle of the same family, a Colymb6tes, once flew on board 
the ' Beagle,' when forty-five miles distant from the nearest land ; 
how much farther it might have been blown by a favouring gale 
no one can tell. 

With respect to plants, it has long been known what enormous 
ranges many fresh-water, and even marsh species, have, both over 
continents and to the most remote oceanic islands. This is strikingly 
illustrated, according to Alph. de Candoiie, in those large groups of 
terrestrial plants, w^iich have very few aquatic members ; for the 
latter seem immediately to acquire, as if in consequence, a wide 
range. I think favourable means of dispersal explain this fact. I 
have before mentioned that earth occasionally adheres in some 
quantity to the feet and beaks of birds, ^\'ading birds, which fre- 
quent the muddy edges of ponds, if suddenly flushed, would be the 
most likely to hav